diff --git a/src/AliasedExpression.php b/src/AliasedExpression.php
index dca03bc..4a14e2e 100644
--- a/src/AliasedExpression.php
+++ b/src/AliasedExpression.php
@@ -7,7 +7,7 @@
 class AliasedExpression extends Expression
 {
     /** @var string */
-    protected $alias;
+    protected string $alias;
 
     /**
      * Create a new database expression
diff --git a/src/Behavior/Binary.php b/src/Behavior/Binary.php
index 43e3e11..75fd294 100644
--- a/src/Behavior/Binary.php
+++ b/src/Behavior/Binary.php
@@ -11,15 +11,13 @@
 use ipl\Stdlib\Filter\Condition;
 use UnexpectedValueException;
 
-use function ipl\Stdlib\get_php_type;
-
 /**
  * Support hex filters for binary columns and PHP resource (in) / bytea hex format (out) transformation for PostgreSQL
  */
 class Binary extends PropertyBehavior implements QueryAwareBehavior, RewriteFilterBehavior
 {
     /** @var bool Whether the query is using a pgsql adapter */
-    protected $isPostgres = true;
+    protected bool $isPostgres = true;
 
     public function fromDb($value, $key, $_)
     {
diff --git a/src/Behavior/BoolCast.php b/src/Behavior/BoolCast.php
index 3163ff9..bc65a0e 100644
--- a/src/Behavior/BoolCast.php
+++ b/src/Behavior/BoolCast.php
@@ -19,20 +19,20 @@
 class BoolCast extends PropertyBehavior
 {
     /** @var mixed Database value for boolean `false` */
-    protected $falseValue = 'n';
+    protected mixed $falseValue = 'n';
 
     /** @var mixed Database value for boolean `true` */
-    protected $trueValue = 'y';
+    protected mixed $trueValue = 'y';
 
     /** @var bool Whether to throw an exception if the value is not equal to the value for false or true */
-    protected $strict = true;
+    protected bool $strict = true;
 
     /**
      * Get the database value representing boolean `false`
      *
      * @return mixed
      */
-    public function getFalseValue()
+    public function getFalseValue(): mixed
     {
         return $this->falseValue;
     }
@@ -44,7 +44,7 @@ public function getFalseValue()
      *
      * @return $this
      */
-    public function setFalseValue($falseValue): self
+    public function setFalseValue(mixed $falseValue): static
     {
         $this->falseValue = $falseValue;
 
@@ -56,7 +56,7 @@ public function setFalseValue($falseValue): self
      *
      * @return mixed
      */
-    public function getTrueValue()
+    public function getTrueValue(): mixed
     {
         return $this->trueValue;
     }
@@ -68,7 +68,7 @@ public function getTrueValue()
      *
      * @return $this
      */
-    public function setTrueValue($trueValue): self
+    public function setTrueValue(mixed $trueValue): static
     {
         $this->trueValue = $trueValue;
 
@@ -92,7 +92,7 @@ public function isStrict(): bool
      *
      * @return $this
      */
-    public function setStrict(bool $strict): self
+    public function setStrict(bool $strict): static
     {
         $this->strict = $strict;
 
diff --git a/src/Behaviors.php b/src/Behaviors.php
index 5c54350..d46a83e 100644
--- a/src/Behaviors.php
+++ b/src/Behaviors.php
@@ -16,32 +16,34 @@
 class Behaviors implements IteratorAggregate
 {
     /** @var array Registered behaviors */
-    protected $behaviors = [];
+    protected array $behaviors = [];
 
     /** @var RetrieveBehavior[] Registered retrieve behaviors */
-    protected $retrieveBehaviors = [];
+    protected array $retrieveBehaviors = [];
 
     /** @var PersistBehavior[] Registered persist behaviors */
-    protected $persistBehaviors = [];
+    protected array $persistBehaviors = [];
 
     /** @var PropertyBehavior[] Registered property behaviors */
-    protected $propertyBehaviors = [];
+    protected array $propertyBehaviors = [];
 
     /** @var RewriteFilterBehavior[] Registered rewrite filter behaviors */
-    protected $rewriteFilterBehaviors = [];
+    protected array $rewriteFilterBehaviors = [];
 
     /** @var RewriteColumnBehavior[] Registered rewrite column behaviors */
-    protected $rewriteColumnBehaviors = [];
+    protected array $rewriteColumnBehaviors = [];
 
     /** @var RewritePathBehavior[] Registered rewrite path behaviors */
-    protected $rewritePathBehaviors = [];
+    protected array $rewritePathBehaviors = [];
 
     /**
      * Add a behavior
      *
-     * @param PersistBehavior|PropertyBehavior|RetrieveBehavior|RewriteFilterBehavior $behavior
+     * @param Behavior $behavior
+     *
+     * @return void
      */
-    public function add(Behavior $behavior)
+    public function add(Behavior $behavior): void
     {
         $this->behaviors[] = $behavior;
 
@@ -86,8 +88,10 @@ public function getIterator(): Traversable
      * Apply all retrieve behaviors on the given model
      *
      * @param Model $model
+     *
+     * @return void
      */
-    public function retrieve(Model $model)
+    public function retrieve(Model $model): void
     {
         foreach ($this->retrieveBehaviors as $behavior) {
             $behavior->retrieve($model);
@@ -98,8 +102,10 @@ public function retrieve(Model $model)
      * Apply all persist behaviors on the given model
      *
      * @param Model $model
+     *
+     * @return void
      */
-    public function persist(Model $model)
+    public function persist(Model $model): void
     {
         foreach ($this->persistBehaviors as $behavior) {
             $behavior->persist($model);
@@ -114,7 +120,7 @@ public function persist(Model $model)
      *
      * @return mixed
      */
-    public function retrieveProperty($value, $key)
+    public function retrieveProperty(mixed $value, string $key): mixed
     {
         foreach ($this->propertyBehaviors as $behavior) {
             $value = $behavior->retrieveProperty($value, $key);
@@ -131,7 +137,7 @@ public function retrieveProperty($value, $key)
      *
      * @return mixed
      */
-    public function persistProperty($value, $key)
+    public function persistProperty(mixed $value, string $key): mixed
     {
         foreach ($this->propertyBehaviors as $behavior) {
             $value = $behavior->persistProperty($value, $key);
@@ -144,11 +150,11 @@ public function persistProperty($value, $key)
      * Rewrite the given filter condition
      *
      * @param Filter\Condition $condition
-     * @param string           $relation Absolute path (with a trailing dot) of the model
+     * @param ?string          $relation Absolute path (with a trailing dot) of the model
      *
      * @return Filter\Rule|null
      */
-    public function rewriteCondition(Filter\Condition $condition, $relation = null)
+    public function rewriteCondition(Filter\Condition $condition, ?string $relation = null): ?Filter\Rule
     {
         $filter = null;
         foreach ($this->rewriteFilterBehaviors as $behavior) {
@@ -164,12 +170,12 @@ public function rewriteCondition(Filter\Condition $condition, $relation = null)
     /**
      * Rewrite the given relation path
      *
-     * @param string $path
-     * @param string $relation Absolute path of the model
+     * @param string  $path
+     * @param ?string $relation Absolute path of the model
      *
      * @return string|null
      */
-    public function rewritePath($path, $relation = null)
+    public function rewritePath(string $path, ?string $relation = null): ?string
     {
         $newPath = null;
         foreach ($this->rewritePathBehaviors as $behavior) {
@@ -185,12 +191,12 @@ public function rewritePath($path, $relation = null)
     /**
      * Rewrite the given column
      *
-     * @param string $column
-     * @param string $relation Absolute path of the model
+     * @param string  $column
+     * @param ?string $relation Absolute path of the model
      *
      * @return mixed
      */
-    public function rewriteColumn($column, $relation = null)
+    public function rewriteColumn(string $column, ?string $relation = null): mixed
     {
         $newColumn = null;
         foreach ($this->rewriteColumnBehaviors as $behavior) {
diff --git a/src/ColumnDefinition.php b/src/ColumnDefinition.php
index ddb8062..b324a97 100644
--- a/src/ColumnDefinition.php
+++ b/src/ColumnDefinition.php
@@ -3,15 +3,14 @@
 namespace ipl\Orm;
 
 use InvalidArgumentException;
-use LogicException;
 
 class ColumnDefinition
 {
     /** @var string The name of the column */
-    protected $name;
+    protected string $name;
 
     /** @var ?string The label of the column */
-    protected $label;
+    protected ?string $label = null;
 
     /**
      * Create a new column definition
@@ -50,7 +49,7 @@ public function getLabel(): ?string
      *
      * @return $this
      */
-    public function setLabel(?string $label): self
+    public function setLabel(?string $label): static
     {
         $this->label = $label;
 
@@ -62,9 +61,11 @@ public function setLabel(?string $label): self
      *
      * @param array $options
      *
-     * @return self
+     * @return static
+     *
+     * @throws InvalidArgumentException If the given options do not provide a name
      */
-    public static function fromArray(array $options): self
+    public static function fromArray(array $options): static
     {
         if (! isset($options['name'])) {
             throw new InvalidArgumentException('$options must provide a name');
diff --git a/src/Common/PropertiesWithDefaults.php b/src/Common/PropertiesWithDefaults.php
index e8d3a84..3ee43da 100644
--- a/src/Common/PropertiesWithDefaults.php
+++ b/src/Common/PropertiesWithDefaults.php
@@ -4,14 +4,15 @@
 
 use Closure;
 use Traversable;
+use ipl\Stdlib\Properties;
 
 trait PropertiesWithDefaults
 {
-    use \ipl\Stdlib\Properties {
-        \ipl\Stdlib\Properties::getProperty as private parentGetProperty;
+    use Properties {
+        Properties::getProperty as private parentGetProperty;
     }
 
-    protected function getProperty($key)
+    protected function getProperty(string $key): mixed
     {
         if (isset($this->properties[$key]) && $this->properties[$key] instanceof Closure) {
             $this->setProperty($key, $this->properties[$key]($this, $key));
diff --git a/src/Common/SortUtil.php b/src/Common/SortUtil.php
index a14ea2b..2a5addf 100644
--- a/src/Common/SortUtil.php
+++ b/src/Common/SortUtil.php
@@ -9,17 +9,17 @@ class SortUtil
     /**
      * Create the sort column(s) and direction(s) from the given sort spec
      *
-     * @param array|string $sort
+     * @param string|array $sort
      *
      * @return array<int, mixed> Sort column(s) and direction(s) suitable for {@link OrderByInterface::orderBy()}
      */
-    public static function createOrderBy($sort): array
+    public static function createOrderBy(string|array $sort): array
     {
         $columnsAndDirections = static::explodeSortSpec($sort);
         $orderBy = [];
 
         foreach ($columnsAndDirections as $columnAndDirection) {
-            list($column, $direction) = static::splitColumnAndDirection($columnAndDirection);
+            [$column, $direction] = static::splitColumnAndDirection($columnAndDirection);
 
             $orderBy[] = [$column, $direction];
         }
@@ -30,11 +30,11 @@ public static function createOrderBy($sort): array
     /**
      * Explode the given sort spec into its sort parts
      *
-     * @param array|string $sort
+     * @param string|array $sort
      *
      * @return array
      */
-    public static function explodeSortSpec($sort)
+    public static function explodeSortSpec(string|array $sort): array
     {
         return Str::trimSplit(implode(',', (array) $sort));
     }
@@ -42,11 +42,11 @@ public static function explodeSortSpec($sort)
     /**
      * Normalize the given sort spec to a sort string
      *
-     * @param array|string $sort
+     * @param string|array $sort
      *
      * @return string
      */
-    public static function normalizeSortSpec($sort)
+    public static function normalizeSortSpec(string|array $sort): string
     {
         return implode(',', static::explodeSortSpec($sort));
     }
@@ -58,7 +58,7 @@ public static function normalizeSortSpec($sort)
      *
      * @return array
      */
-    public static function splitColumnAndDirection($sort)
+    public static function splitColumnAndDirection(string $sort): array
     {
         return Str::symmetricSplit($sort, ' ', 2);
     }
diff --git a/src/Compat/FilterProcessor.php b/src/Compat/FilterProcessor.php
index 1b3aab9..f4f215e 100644
--- a/src/Compat/FilterProcessor.php
+++ b/src/Compat/FilterProcessor.php
@@ -20,9 +20,9 @@
 
 class FilterProcessor extends \ipl\Sql\Compat\FilterProcessor
 {
-    protected $baseJoins = [];
+    protected array $baseJoins = [];
 
-    protected $madeJoins = [];
+    protected array $madeJoins = [];
 
     /**
      * Require and resolve the filter rule and apply it on the query
@@ -33,8 +33,10 @@ class FilterProcessor extends \ipl\Sql\Compat\FilterProcessor
      *
      * @param Filter\Rule $filter
      * @param Query $query
+     *
+     * @return void
      */
-    public static function apply(Filter\Rule $filter, Query $query)
+    public static function apply(Filter\Rule $filter, Query $query): void
     {
         if ($query instanceof UnionQuery) {
             foreach ($query->getUnions() as $union) {
@@ -71,7 +73,7 @@ public static function apply(Filter\Rule $filter, Query $query)
      *
      * @return void
      */
-    public static function resolveFilter(Filter\Chain $filter, Query $query)
+    public static function resolveFilter(Filter\Chain $filter, Query $query): void
     {
         $processor = new static();
         foreach ($query->getUtilize() as $path => $_) {
@@ -81,8 +83,11 @@ public static function resolveFilter(Filter\Chain $filter, Query $query)
         $processor->requireAndResolveFilterColumns($filter, $query);
     }
 
-    protected function requireAndResolveFilterColumns(Filter\Rule $filter, Query $query, $forceOptimization = null)
-    {
+    protected function requireAndResolveFilterColumns(
+        Filter\Rule $filter,
+        Query $query,
+        ?bool $forceOptimization = null
+    ): MetaDataProvider|Filter\Rule|null {
         if ($filter instanceof Filter\Condition) {
             if (
                 $filter instanceof Exists
@@ -90,7 +95,7 @@ protected function requireAndResolveFilterColumns(Filter\Rule $filter, Query $qu
                 || $filter instanceof In
                 || $filter instanceof NotIn
             ) {
-                return;
+                return null;
             }
 
             $resolver = $query->getResolver();
@@ -102,7 +107,7 @@ protected function requireAndResolveFilterColumns(Filter\Rule $filter, Query $qu
 
             $filter->metaData()->set('columnPath', $column);
 
-            list($relationPath, $columnName) = preg_split('/\.(?=[^.]+$)/', $column);
+            [$relationPath, $columnName] = preg_split('/\.(?=[^.]+$)/', $column);
 
             $subject = null;
             $relations = new AppendIterator();
@@ -270,7 +275,7 @@ protected function requireAndResolveFilterColumns(Filter\Rule $filter, Query $qu
                     $subQueryFilters[] = [$baseFilters ?: $generalRules, $count, false];
                 }
 
-                foreach ($subQueryFilters as list($filters, $count, $negate)) {
+                foreach ($subQueryFilters as [$filters, $count, $negate]) {
                     $subQueryFilter = null;
                     if ($count !== null) {
                         $aggregateFilter = Filter::any();
@@ -387,5 +392,7 @@ function ($relationFilter) use ($rule) {
                 $filter->remove($rule);
             }
         }
+
+        return null;
     }
 }
diff --git a/src/Contract/PersistBehavior.php b/src/Contract/PersistBehavior.php
index a6db05d..50a6952 100644
--- a/src/Contract/PersistBehavior.php
+++ b/src/Contract/PersistBehavior.php
@@ -14,5 +14,5 @@ interface PersistBehavior extends Behavior
      *
      * @param Model $model
      */
-    public function persist(Model $model);
+    public function persist(Model $model): void;
 }
diff --git a/src/Contract/PropertyBehavior.php b/src/Contract/PropertyBehavior.php
index c828458..6242c20 100644
--- a/src/Contract/PropertyBehavior.php
+++ b/src/Contract/PropertyBehavior.php
@@ -8,7 +8,7 @@
 abstract class PropertyBehavior implements RetrieveBehavior, PersistBehavior
 {
     /** @var array Property names of which the value should be processed */
-    protected $properties;
+    protected array $properties;
 
     /**
      * PropertyBehavior constructor
@@ -24,7 +24,7 @@ public function __construct(array $properties)
         }
     }
 
-    public function retrieve(Model $model)
+    public function retrieve(Model $model): void
     {
         foreach ($this->properties as $key => $ctx) {
             if ($model->hasProperty($key)) {
@@ -33,7 +33,7 @@ public function retrieve(Model $model)
         }
     }
 
-    public function persist(Model $model)
+    public function persist(Model $model): void
     {
         foreach ($this->properties as $key => $ctx) {
             try {
@@ -52,7 +52,7 @@ public function persist(Model $model)
      *
      * @return mixed
      */
-    public function retrieveProperty($value, $key)
+    public function retrieveProperty(mixed $value, string $key): mixed
     {
         if (! isset($this->properties[$key])) {
             return $value;
@@ -69,7 +69,7 @@ public function retrieveProperty($value, $key)
      *
      * @return mixed
      */
-    public function persistProperty($value, $key)
+    public function persistProperty(mixed $value, string $key): mixed
     {
         if (! isset($this->properties[$key])) {
             return $value;
diff --git a/src/Contract/RetrieveBehavior.php b/src/Contract/RetrieveBehavior.php
index 884d074..be3c037 100644
--- a/src/Contract/RetrieveBehavior.php
+++ b/src/Contract/RetrieveBehavior.php
@@ -14,5 +14,5 @@ interface RetrieveBehavior extends Behavior
      *
      * @param Model $model
      */
-    public function retrieve(Model $model);
+    public function retrieve(Model $model): void;
 }
diff --git a/src/Defaults.php b/src/Defaults.php
index aa2d517..15deab0 100644
--- a/src/Defaults.php
+++ b/src/Defaults.php
@@ -8,7 +8,7 @@
 class Defaults implements IteratorAggregate
 {
     /** @var array<string, mixed> Registered defaults */
-    protected $defaults = [];
+    protected array $defaults = [];
 
     /**
      * Iterate over the defaults
@@ -31,7 +31,7 @@ public function getIterator(): Traversable
      *
      * @return $this
      */
-    public function add(string $property, $default): self
+    public function add(string $property, mixed $default): static
     {
         $this->defaults[$property] = $default;
 
diff --git a/src/Exception/InvalidColumnException.php b/src/Exception/InvalidColumnException.php
index cd320c6..8fb9344 100644
--- a/src/Exception/InvalidColumnException.php
+++ b/src/Exception/InvalidColumnException.php
@@ -8,10 +8,10 @@
 class InvalidColumnException extends Exception
 {
     /** @var string The column name */
-    protected $column;
+    protected string $column;
 
     /** @var Model The target model */
-    protected $model;
+    protected Model $model;
 
     /**
      * Create a new InvalidColumnException
@@ -19,9 +19,9 @@ class InvalidColumnException extends Exception
      * @param string $column The column name
      * @param Model $model The target model
      */
-    public function __construct($column, Model $model)
+    public function __construct(string $column, Model $model)
     {
-        $this->column = (string) $column;
+        $this->column = $column;
         $this->model = $model;
 
         parent::__construct(sprintf(
@@ -36,7 +36,7 @@ public function __construct($column, Model $model)
      *
      * @return string
      */
-    public function getColumn()
+    public function getColumn(): string
     {
         return $this->column;
     }
@@ -46,7 +46,7 @@ public function getColumn()
      *
      * @return Model
      */
-    public function getModel()
+    public function getModel(): Model
     {
         return $this->model;
     }
diff --git a/src/Exception/InvalidRelationException.php b/src/Exception/InvalidRelationException.php
index 215b704..6cfe121 100644
--- a/src/Exception/InvalidRelationException.php
+++ b/src/Exception/InvalidRelationException.php
@@ -8,10 +8,10 @@
 class InvalidRelationException extends Exception
 {
     /** @var string The relation name */
-    protected $relation;
+    protected string $relation;
 
-    /** @var Model The target model */
-    protected $model;
+    /** @var ?Model The target model */
+    protected ?Model $model = null;
 
     /**
      * Create a new InvalidRelationException
@@ -19,9 +19,9 @@ class InvalidRelationException extends Exception
      * @param string $relation The relation name
      * @param ?Model $model The target model
      */
-    public function __construct($relation, ?Model $model = null)
+    public function __construct(string $relation, ?Model $model = null)
     {
-        $this->relation = (string) $relation;
+        $this->relation = $relation;
         $this->model = $model;
 
         parent::__construct(sprintf(
@@ -36,7 +36,7 @@ public function __construct($relation, ?Model $model = null)
      *
      * @return string
      */
-    public function getRelation()
+    public function getRelation(): string
     {
         return $this->relation;
     }
@@ -44,9 +44,9 @@ public function getRelation()
     /**
      * Get the target model
      *
-     * @return Model
+     * @return ?Model
      */
-    public function getModel()
+    public function getModel(): ?Model
     {
         return $this->model;
     }
diff --git a/src/Hydrator.php b/src/Hydrator.php
index 63b3ff2..827bac4 100644
--- a/src/Hydrator.php
+++ b/src/Hydrator.php
@@ -2,8 +2,8 @@
 
 namespace ipl\Orm;
 
-use InvalidArgumentException;
 use ipl\Orm\Exception\InvalidRelationException;
+use InvalidArgumentException;
 
 /**
  * Hydrates raw database rows into concrete model instances.
@@ -11,13 +11,13 @@
 class Hydrator
 {
     /** @var array Additional hydration rules for the model's relations */
-    protected $hydrators = [];
+    protected array $hydrators = [];
 
     /** @var array<string, array<string, bool>> Map of columns to referencing paths */
-    protected $columnToTargetMap = [];
+    protected array $columnToTargetMap = [];
 
     /** @var Query The query the hydration rules are for */
-    protected $query;
+    protected Query $query;
 
     /**
      * Create a new Hydrator
@@ -36,12 +36,12 @@ public function __construct(Query $query)
      *
      * @return $this
      *
-     * @throws \InvalidArgumentException If a hydrator for the given path already exists
+     * @throws InvalidArgumentException If a hydrator for the given path already exists
      */
-    public function add($path)
+    public function add(string $path): static
     {
         if (isset($this->hydrators[$path])) {
-            throw new \InvalidArgumentException("Hydrator for path '$path' already exists");
+            throw new InvalidArgumentException("Hydrator for path '$path' already exists");
         }
 
         $resolver = $this->query->getResolver();
@@ -106,12 +106,12 @@ protected function updateColumnToTargetMap(string $path, array $columnToProperty
      *
      * @return Model
      */
-    public function hydrate(array $data, Model $model)
+    public function hydrate(array $data, Model $model): Model
     {
         $defaultsToApply = [];
         $columnToTargetMap = $this->columnToTargetMap;
         foreach ($this->hydrators as $path => $vars) {
-            list($target, $relation, $columnToPropertyMap, $defaults) = $vars;
+            [$target, $relation, $columnToPropertyMap, $defaults] = $vars;
 
             $subject = $model;
             if ($relation !== null) {
@@ -200,7 +200,7 @@ public function hydrate(array $data, Model $model)
         }
 
         // Apply defaults last, otherwise we may evaluate them during hydration
-        foreach ($defaultsToApply as list($subject, $defaults)) {
+        foreach ($defaultsToApply as [$subject, $defaults]) {
             foreach ($defaults as $name => $default) {
                 if (! $subject->hasProperty($name)) {
                     $subject->$name = $default;
@@ -221,8 +221,12 @@ public function hydrate(array $data, Model $model)
      *
      * @return array
      */
-    protected function extractAndMap(array &$data, array $columnToPropertyMap, string $path, array &$columnToTargetMap)
-    {
+    protected function extractAndMap(
+        array &$data,
+        array $columnToPropertyMap,
+        string $path,
+        array &$columnToTargetMap
+    ): array {
         $extracted = [];
         foreach (array_intersect_key($columnToPropertyMap, $data) as $column => $property) {
             $extracted[$property] = $data[$column];
diff --git a/src/Query.php b/src/Query.php
index 76cd8d3..14896d4 100644
--- a/src/Query.php
+++ b/src/Query.php
@@ -47,44 +47,44 @@ class Query implements Filterable, LimitOffsetInterface, OrderByInterface, Pagin
      */
     public const ON_SELECT_ASSEMBLED = 'selectAssembled';
 
-    /** @var int Count cache */
-    protected $count;
+    /** @var ?int Count cache */
+    protected ?int $count = null;
 
     /** @var Connection Database connection */
     protected $db;
 
     /** @var string Class to return results as */
-    protected $resultSetClass = ResultSet::class;
+    protected string $resultSetClass = ResultSet::class;
 
     /** @var Model Model to query */
     protected $model;
 
     /** @var array Columns to select from the model (or its relations). If empty, all columns are selected */
-    protected $columns = [];
+    protected array $columns = [];
 
     /** @var array Additional columns to select from the model (or its relations) */
-    protected $withColumns = [];
+    protected array $withColumns = [];
 
     /** @var array Columns not to select from the model (or its relations) */
-    protected $withoutColumns = [];
+    protected array $withoutColumns = [];
 
     /** @var bool Whether to peek ahead for more results */
-    protected $peekAhead = false;
+    protected bool $peekAhead = false;
 
-    /** @var Resolver Column and relation resolver */
-    protected $resolver;
+    /** @var ?Resolver Column and relation resolver */
+    protected ?Resolver $resolver = null;
 
-    /** @var Select Base SELECT query */
-    protected $selectBase;
+    /** @var ?Select Base SELECT query */
+    protected ?Select $selectBase = null;
 
     /** @var Relation[] Relations to eager load */
-    protected $with = [];
+    protected array $with = [];
 
     /** @var Relation[] Relations to utilize (join) */
-    protected $utilize = [];
+    protected array $utilize = [];
 
     /** @var bool Whether to disable the default sorts of the model */
-    protected $disableDefaultSort = false;
+    protected bool $disableDefaultSort = false;
 
     /**
      * Get the database connection
@@ -103,7 +103,7 @@ public function getDb()
      *
      * @return $this
      */
-    public function setDb(Connection $db)
+    public function setDb(Connection $db): static
     {
         $this->db = $db;
 
@@ -115,7 +115,7 @@ public function setDb(Connection $db)
      *
      * @return string
      */
-    public function getResultSetClass()
+    public function getResultSetClass(): string
     {
         return $this->resultSetClass;
     }
@@ -129,12 +129,8 @@ public function getResultSetClass()
      *
      * @throws InvalidArgumentException If class is not an instance of {@link ResultSet}
      */
-    public function setResultSetClass($class)
+    public function setResultSetClass(string $class): static
     {
-        if (! is_string($class)) {
-            throw new InvalidArgumentException('Argument $class must be a string');
-        }
-
         if (! (new ReflectionClass($class))->newInstanceWithoutConstructor() instanceof ResultSet) {
             throw new InvalidArgumentException(
                 $class . ' must be an instance of ' . ResultSet::class
@@ -159,11 +155,11 @@ public function getModel()
     /**
      * Set the model to query
      *
-     * @param $model
+     * @param Model $model
      *
      * @return $this
      */
-    public function setModel(Model $model)
+    public function setModel(Model $model): static
     {
         $this->model = $model;
         $this->getResolver()->setAlias($model, $model->getTableAlias());
@@ -176,7 +172,7 @@ public function setModel(Model $model)
      *
      * @return array
      */
-    public function getColumns()
+    public function getColumns(): array
     {
         return $this->columns;
     }
@@ -188,7 +184,7 @@ public function getColumns()
      *
      * @return $this
      */
-    public function setFilter(Filter\Chain $filter)
+    public function setFilter(Filter\Chain $filter): static
     {
         $this->filter = $filter;
 
@@ -204,9 +200,9 @@ public function setFilter(Filter\Chain $filter)
      *
      * @return $this
      */
-    public function disableDefaultSort($disable = true)
+    public function disableDefaultSort(bool $disable = true): static
     {
-        $this->disableDefaultSort = (bool) $disable;
+        $this->disableDefaultSort = $disable;
 
         return $this;
     }
@@ -216,7 +212,7 @@ public function disableDefaultSort($disable = true)
      *
      * @return bool
      */
-    public function defaultSortDisabled()
+    public function defaultSortDisabled(): bool
     {
         return $this->disableDefaultSort;
     }
@@ -236,7 +232,7 @@ public function defaultSortDisabled()
      *
      * @return $this
      */
-    public function columns($columns)
+    public function columns(string|array $columns): static
     {
         $this->columns = (array) $columns;
         $this->withColumns = [];
@@ -255,7 +251,7 @@ public function columns($columns)
      *
      * @return $this
      */
-    public function withColumns($columns)
+    public function withColumns(string|array $columns): static
     {
         $tableName = $this->getModel()->getTableAlias();
 
@@ -283,7 +279,7 @@ public function withColumns($columns)
      *
      * @return $this
      */
-    public function withoutColumns($columns): self
+    public function withoutColumns(string|array $columns): static
     {
         $tableName = $this->getModel()->getTableAlias();
 
@@ -304,7 +300,7 @@ public function withoutColumns($columns): self
      *
      * @return Resolver
      */
-    public function getResolver()
+    public function getResolver(): Resolver
     {
         if ($this->resolver === null) {
             $this->resolver = new Resolver($this);
@@ -318,7 +314,7 @@ public function getResolver()
      *
      * @return Select
      */
-    public function getSelectBase()
+    public function getSelectBase(): Select
     {
         if ($this->selectBase === null) {
             $this->selectBase = new Select();
@@ -336,7 +332,7 @@ public function getSelectBase()
      *
      * @return Relation[]
      */
-    public function getWith()
+    public function getWith(): array
     {
         return $this->with;
     }
@@ -348,7 +344,7 @@ public function getWith()
      *
      * @return $this
      */
-    public function with($relations)
+    public function with(string|array $relations): static
     {
         $tableName = $this->getModel()->getTableAlias();
         foreach ((array) $relations as $relation) {
@@ -366,7 +362,7 @@ public function with($relations)
      *
      * @return $this
      */
-    public function without($relations)
+    public function without(string|array $relations): static
     {
         $tableName = $this->getModel()->getTableAlias();
         foreach ((array) $relations as $relation) {
@@ -382,7 +378,7 @@ public function without($relations)
      *
      * @return Relation[]
      */
-    public function getUtilize()
+    public function getUtilize(): array
     {
         return $this->utilize;
     }
@@ -394,7 +390,7 @@ public function getUtilize()
      *
      * @return $this
      */
-    public function utilize($path)
+    public function utilize(string $path): static
     {
         $path = $this->getResolver()->qualifyPath($path, $this->getModel()->getTableAlias());
         $this->utilize[$path] = $this->getResolver()->resolveRelation($path);
@@ -409,7 +405,7 @@ public function utilize($path)
      *
      * @return $this
      */
-    public function omit($path)
+    public function omit(string $path): static
     {
         $path = $this->getResolver()->qualifyPath($path, $this->getModel()->getTableAlias());
         unset($this->utilize[$path]);
@@ -422,7 +418,7 @@ public function omit($path)
      *
      * @return Select
      */
-    public function assembleSelect()
+    public function assembleSelect(): Select
     {
         $columns = $this->getColumns();
         $model = $this->getModel();
@@ -500,7 +496,7 @@ public function assembleSelect()
                     continue;
                 }
 
-                foreach ($relation->resolve() as list($source, $target, $relatedKeys)) {
+                foreach ($relation->resolve() as [$source, $target, $relatedKeys]) {
                     /** @var Model $source */
                     /** @var Model $target */
 
@@ -560,7 +556,7 @@ public function assembleSelect()
      *
      * @return Hydrator
      */
-    public function createHydrator()
+    public function createHydrator(): Hydrator
     {
         $hydrator = new Hydrator($this);
 
@@ -582,7 +578,7 @@ public function createHydrator()
      *
      * @throws InvalidArgumentException If the relation with the given name does not exist
      */
-    public function derive($relation, Model $source)
+    public function derive($relation, Model $source): static
     {
         // TODO: Think of a way to merge derive() and createSubQuery()
         return $this->createSubQuery(
@@ -602,7 +598,7 @@ public function derive($relation, Model $source)
      *
      * @return static
      */
-    public function createSubQuery(Model $target, $targetPath, ?Model $from = null, bool $link = true)
+    public function createSubQuery(Model $target, string $targetPath, ?Model $from = null, bool $link = true): static
     {
         $subQuery = (new static())
             ->setDb($this->getDb())
@@ -654,7 +650,7 @@ public function createSubQuery(Model $target, $targetPath, ?Model $from = null,
      *
      * @return array
      */
-    public function dump()
+    public function dump(): array
     {
         return $this->getDb()->getQueryBuilder()->assembleSelect($this->assembleSelect());
     }
@@ -664,7 +660,7 @@ public function dump()
      *
      * @return ResultSet
      */
-    public function execute()
+    public function execute(): ResultSet
     {
         $class = $this->getResultSetClass();
         /** @var ResultSet $class Just for type hinting. $class is of course a string */
@@ -677,7 +673,7 @@ public function execute()
      *
      * @return Model|null Null in case there's no result
      */
-    public function first()
+    public function first(): ?Model
     {
         return $this->execute()->current();
     }
@@ -692,9 +688,9 @@ public function first()
      *
      * @return $this
      */
-    public function peekAhead($peekAhead = true)
+    public function peekAhead(bool $peekAhead = true): static
     {
-        $this->peekAhead = (bool) $peekAhead;
+        $this->peekAhead = $peekAhead;
 
         return $this;
     }
@@ -702,9 +698,9 @@ public function peekAhead($peekAhead = true)
     /**
      * Yield the query's results
      *
-     * @return \Generator
+     * @return Generator
      */
-    public function yieldResults()
+    public function yieldResults(): Generator
     {
         $select = $this->assembleSelect();
         $stmt = $this->getDb()->select($select);
@@ -739,11 +735,11 @@ public function getIterator(): Traversable
      *
      * @return SplObjectStorage
      */
-    protected function groupColumnsByTarget(Generator $columns)
+    protected function groupColumnsByTarget(Generator $columns): SplObjectStorage
     {
         $columnStorage = new SplObjectStorage();
 
-        foreach ($columns as list($target, $alias, $column)) {
+        foreach ($columns as [$target, $alias, $column]) {
             if (! $columnStorage->offsetExists($target)) {
                 $resolved = new ArrayObject();
                 $columnStorage->offsetSet($target, $resolved);
@@ -768,7 +764,7 @@ protected function groupColumnsByTarget(Generator $columns)
      *
      * @return $this
      */
-    protected function order(Select $select)
+    protected function order(Select $select): static
     {
         $orderBy = $this->getOrderBy();
         $defaultSort = [];
@@ -791,7 +787,7 @@ protected function order(Select $select)
         $selectedColumns = $select->getColumns();
 
         foreach ($orderBy as $part) {
-            list($column, $direction) = $part;
+            [$column, $direction] = $part;
 
             if (! $column instanceof ExpressionInterface && isset($selectedColumns[$column])) {
                 // If it's a custom alias, we have no other way of knowing it,
@@ -804,7 +800,7 @@ protected function order(Select $select)
             }
         }
 
-        foreach ($resolver->requireAndResolveColumns($columns) as list($model, $alias, $column)) {
+        foreach ($resolver->requireAndResolveColumns($columns) as [$model, $alias, $column]) {
             $direction = array_shift($directions);
             $selectColumns = $resolver->getSelectColumns($model);
             $tableName = $resolver->getAlias($model);
diff --git a/src/Relation.php b/src/Relation.php
index 1959363..2aca85c 100644
--- a/src/Relation.php
+++ b/src/Relation.php
@@ -2,7 +2,8 @@
 
 namespace ipl\Orm;
 
-use function ipl\Stdlib\get_php_type;
+use Generator;
+use UnexpectedValueException;
 
 /**
  * Relations represent the connection between models, i.e. the association between rows in one or more tables
@@ -16,11 +17,11 @@ class Relation
     /** @var Model Source model */
     protected $source;
 
-    /** @var string|array Column name(s) of the foreign key found in the target table */
-    protected $foreignKey;
+    /** @var string|array|null Column name(s) of the foreign key found in the target table */
+    protected string|array|null $foreignKey = null;
 
-    /** @var string|array Column name(s) of the candidate key in the source table which references the foreign key */
-    protected $candidateKey;
+    /** @var string|array|null Column name(s) of the candidate key in the source table which references the foreign key */
+    protected string|array|null $candidateKey = null;
 
     /** @var string Target model class */
     protected $targetClass;
@@ -29,13 +30,13 @@ class Relation
     protected $target;
 
     /** @var string Type of the JOIN used in the query */
-    protected $joinType = 'INNER';
+    protected string $joinType = 'INNER';
 
     /** @var bool Whether this is the inverse of a relationship */
-    protected $inverse;
+    protected bool $inverse = false;
 
     /** @var bool Whether this is a to-one relationship */
-    protected $isOne = true;
+    protected bool $isOne = true;
 
     /**
      * Get the default column name(s) in the source table used to match the foreign key
@@ -46,7 +47,7 @@ class Relation
      *
      * @return array
      */
-    public static function getDefaultCandidateKey(Model $source)
+    public static function getDefaultCandidateKey(Model $source): array
     {
         return (array) $source->getKeyName();
     }
@@ -60,7 +61,7 @@ public static function getDefaultCandidateKey(Model $source)
      *
      * @return array
      */
-    public static function getDefaultForeignKey(Model $source)
+    public static function getDefaultForeignKey(Model $source): array
     {
         $tableName = $source->getTableName();
 
@@ -77,7 +78,7 @@ function ($key) use ($tableName) {
      *
      * @return bool
      */
-    public function isOne()
+    public function isOne(): bool
     {
         return $this->isOne;
     }
@@ -99,7 +100,7 @@ public function getName()
      *
      * @return $this
      */
-    public function setName($name)
+    public function setName(string $name): static
     {
         $this->name = $name;
 
@@ -123,7 +124,7 @@ public function getSource()
      *
      * @return $this
      */
-    public function setSource(Model $source)
+    public function setSource(Model $source): static
     {
         $this->source = $source;
 
@@ -133,9 +134,9 @@ public function setSource(Model $source)
     /**
      * Get the column name(s) of the foreign key found in the target table
      *
-     * @return string|array Array if the foreign key is compound, string otherwise
+     * @return string|array|null Array if the foreign key is compound, string otherwise
      */
-    public function getForeignKey()
+    public function getForeignKey(): string|array|null
     {
         return $this->foreignKey;
     }
@@ -143,11 +144,11 @@ public function getForeignKey()
     /**
      * Set the column name(s) of the foreign key found in the target table
      *
-     * @param string|array $foreignKey Array if the foreign key is compound, string otherwise
+     * @param string|array|null $foreignKey Array if the foreign key is compound, string otherwise
      *
      * @return $this
      */
-    public function setForeignKey($foreignKey)
+    public function setForeignKey(string|array|null $foreignKey): static
     {
         $this->foreignKey = $foreignKey;
 
@@ -157,9 +158,9 @@ public function setForeignKey($foreignKey)
     /**
      * Get the column name(s) of the candidate key in the source table which references the foreign key
      *
-     * @return string|array Array if the candidate key is compound, string otherwise
+     * @return string|array|null Array if the candidate key is compound, string otherwise
      */
-    public function getCandidateKey()
+    public function getCandidateKey(): string|array|null
     {
         return $this->candidateKey;
     }
@@ -167,11 +168,11 @@ public function getCandidateKey()
     /**
      * Set the column name(s) of the candidate key in the source table which references the foreign key
      *
-     * @param string|array $candidateKey Array if the candidate key is compound, string otherwise
+     * @param string|array|null $candidateKey Array if the candidate key is compound, string otherwise
      *
      * @return $this
      */
-    public function setCandidateKey($candidateKey)
+    public function setCandidateKey(string|array|null $candidateKey): static
     {
         $this->candidateKey = $candidateKey;
 
@@ -194,21 +195,9 @@ public function getTargetClass()
      * @param string $targetClass
      *
      * @return $this
-     *
-     * @throws \InvalidArgumentException If the target model class is not of type string
      */
-    public function setTargetClass($targetClass)
+    public function setTargetClass(string $targetClass): static
     {
-        if (! is_string($targetClass)) {
-            // Require a class name here instead of a concrete model in oder to prevent circular references when
-            // constructing relations
-            throw new \InvalidArgumentException(sprintf(
-                '%s() expects parameter 1 to be string, %s given',
-                __METHOD__,
-                get_php_type($targetClass)
-            ));
-        }
-
         $this->targetClass = $targetClass;
 
         return $this;
@@ -239,7 +228,7 @@ public function getTarget()
      *
      * @return $this
      */
-    public function setTarget(Model $target)
+    public function setTarget(Model $target): static
     {
         $this->target = $target;
 
@@ -251,7 +240,7 @@ public function setTarget(Model $target)
      *
      * @return string
      */
-    public function getJoinType()
+    public function getJoinType(): string
     {
         return $this->joinType;
     }
@@ -263,7 +252,7 @@ public function getJoinType()
      *
      * @return Relation
      */
-    public function setJoinType($joinType)
+    public function setJoinType(string $joinType): static
     {
         $this->joinType = $joinType;
 
@@ -277,10 +266,10 @@ public function setJoinType($joinType)
      *
      * @return array Candidate key-foreign key column name pairs
      *
-     * @throws \UnexpectedValueException If there's no candidate key to be found
+     * @throws UnexpectedValueException If there's no candidate key to be found
      *                                   or the foreign key count does not match the candidate key count
      */
-    public function determineKeys(Model $source)
+    public function determineKeys(Model $source): array
     {
         $candidateKey = (array) $this->getCandidateKey();
 
@@ -291,7 +280,7 @@ public function determineKeys(Model $source)
         }
 
         if (empty($candidateKey)) {
-            throw new \UnexpectedValueException(sprintf(
+            throw new UnexpectedValueException(sprintf(
                 "Can't join relation '%s' in model '%s'. No candidate key found.",
                 $this->getName(),
                 get_class($source)
@@ -307,7 +296,7 @@ public function determineKeys(Model $source)
         }
 
         if (count($foreignKey) !== count($candidateKey)) {
-            throw new \UnexpectedValueException(sprintf(
+            throw new UnexpectedValueException(sprintf(
                 "Can't join relation '%s' in model '%s'."
                 . " Foreign key count (%s) does not match candidate key count (%s).",
                 $this->getName(),
@@ -325,9 +314,9 @@ public function determineKeys(Model $source)
      *
      * Yields a three-element array consisting of the source model, target model and the join keys.
      *
-     * @return \Generator
+     * @return Generator
      */
-    public function resolve()
+    public function resolve(): Generator
     {
         $source = $this->getSource();
 
diff --git a/src/Relation/BelongsTo.php b/src/Relation/BelongsTo.php
index 1edcff3..1982f10 100644
--- a/src/Relation/BelongsTo.php
+++ b/src/Relation/BelongsTo.php
@@ -9,5 +9,5 @@
  */
 class BelongsTo extends Relation
 {
-    protected $inverse = true;
+    protected bool $inverse = true;
 }
diff --git a/src/Relation/BelongsToMany.php b/src/Relation/BelongsToMany.php
index fa69245..edf5bf1 100644
--- a/src/Relation/BelongsToMany.php
+++ b/src/Relation/BelongsToMany.php
@@ -2,13 +2,12 @@
 
 namespace ipl\Orm\Relation;
 
+use Generator;
 use ipl\Orm\Model;
 use ipl\Orm\Relation;
 use ipl\Orm\Relations;
 use LogicException;
 
-use function ipl\Stdlib\get_php_type;
-
 /**
  * Many-to-many relationship
  */
@@ -17,22 +16,22 @@ class BelongsToMany extends Relation
     /** @var string Relation class */
     protected const RELATION_CLASS = HasMany::class;
 
-    protected $isOne = false;
+    protected bool $isOne = false;
 
-    /** @var string Name of the join table or junction model class */
-    protected $throughClass;
+    /** @var ?string Name of the join table or junction model class */
+    protected ?string $throughClass = null;
 
     /** @var ?string Alias for the join table or junction model class */
     protected ?string $throughAlias = null;
 
-    /** @var Model The junction model */
-    protected $through;
+    /** @var ?Model The junction model */
+    protected ?Model $through = null;
 
-    /** @var string|array Column name(s) of the target model's foreign key found in the join table */
-    protected $targetForeignKey;
+    /** @var string|array|null Column name(s) of the target model's foreign key found in the join table */
+    protected string|array|null $targetForeignKey = null;
 
-    /** @var string|array Candidate key column name(s) in the target table which references the target foreign key */
-    protected $targetCandidateKey;
+    /** @var string|array|null Candidate key column name(s) in the target table which references the target foreign key */
+    protected string|array|null $targetCandidateKey = null;
 
     /**
      * Get the name of the join table or junction model class
@@ -51,7 +50,7 @@ public function getThroughClass(): ?string
      *
      * @return $this
      */
-    public function through(string $through): self
+    public function through(string $through): static
     {
         $this->throughClass = $through;
 
@@ -75,7 +74,7 @@ public function getThroughAlias(): string
      *
      * @return $this
      */
-    public function setThroughAlias(string $throughAlias): self
+    public function setThroughAlias(string $throughAlias): static
     {
         $this->throughAlias = $throughAlias;
 
@@ -85,7 +84,9 @@ public function setThroughAlias(string $throughAlias): self
     /**
      * Get the junction model
      *
-     * @return Model|Junction
+     * @return Model
+     *
+     * @throws LogicException If no through class is configured
      */
     public function getThrough(): Model
     {
@@ -116,7 +117,7 @@ public function getThrough(): Model
      *
      * @return $this
      */
-    public function setThrough(Model $through): self
+    public function setThrough(Model $through): static
     {
         $this->through = $through;
 
@@ -126,9 +127,9 @@ public function setThrough(Model $through): self
     /**
      * Get the column name(s) of the target model's foreign key found in the join table
      *
-     * @return string|array Array if the foreign key is compound, string otherwise
+     * @return string|array|null Array if the foreign key is compound, string otherwise
      */
-    public function getTargetForeignKey()
+    public function getTargetForeignKey(): string|array|null
     {
         return $this->targetForeignKey;
     }
@@ -140,7 +141,7 @@ public function getTargetForeignKey()
      *
      * @return $this
      */
-    public function setTargetForeignKey($targetForeignKey): self
+    public function setTargetForeignKey(string|array $targetForeignKey): static
     {
         $this->targetForeignKey = $targetForeignKey;
 
@@ -150,9 +151,9 @@ public function setTargetForeignKey($targetForeignKey): self
     /**
      * Get the candidate key column name(s) in the target table which references the target foreign key
      *
-     * @return string|array Array if the foreign key is compound, string otherwise
+     * @return string|array|null Array if the foreign key is compound, string otherwise
      */
-    public function getTargetCandidateKey()
+    public function getTargetCandidateKey(): string|array|null
     {
         return $this->targetCandidateKey;
     }
@@ -164,14 +165,14 @@ public function getTargetCandidateKey()
      *
      * @return $this
      */
-    public function setTargetCandidateKey($targetCandidateKey): self
+    public function setTargetCandidateKey(string|array $targetCandidateKey): static
     {
         $this->targetCandidateKey = $targetCandidateKey;
 
         return $this;
     }
 
-    public function resolve()
+    public function resolve(): Generator
     {
         $source = $this->getSource();
 
@@ -229,7 +230,7 @@ public function resolve()
         }
     }
 
-    protected function extractKey(array $possibleKey)
+    protected function extractKey(array $possibleKey): string|array|null
     {
         $filtered = array_filter($possibleKey);
 
diff --git a/src/Relation/BelongsToOne.php b/src/Relation/BelongsToOne.php
index afdfbec..5272bf3 100644
--- a/src/Relation/BelongsToOne.php
+++ b/src/Relation/BelongsToOne.php
@@ -9,5 +9,5 @@ class BelongsToOne extends BelongsToMany
 {
     protected const RELATION_CLASS = HasOne::class;
 
-    protected $isOne = true;
+    protected bool $isOne = true;
 }
diff --git a/src/Relation/HasMany.php b/src/Relation/HasMany.php
index 3e71e25..13d1abc 100644
--- a/src/Relation/HasMany.php
+++ b/src/Relation/HasMany.php
@@ -9,5 +9,5 @@
  */
 class HasMany extends Relation
 {
-    protected $isOne = false;
+    protected bool $isOne = false;
 }
diff --git a/src/Relations.php b/src/Relations.php
index e19306e..e924096 100644
--- a/src/Relations.php
+++ b/src/Relations.php
@@ -3,6 +3,7 @@
 namespace ipl\Orm;
 
 use ArrayIterator;
+use InvalidArgumentException;
 use ipl\Orm\Exception\InvalidRelationException;
 use ipl\Orm\Relation\BelongsTo;
 use ipl\Orm\Relation\BelongsToMany;
@@ -20,7 +21,7 @@
 class Relations implements IteratorAggregate
 {
     /** @var Relation[] */
-    protected $relations = [];
+    protected array $relations = [];
 
     /**
      * Get whether a relation with the given name exists
@@ -29,7 +30,7 @@ class Relations implements IteratorAggregate
      *
      * @return bool
      */
-    public function has($name)
+    public function has(string $name): bool
     {
         return isset($this->relations[$name]);
     }
@@ -41,9 +42,9 @@ public function has($name)
      *
      * @return Relation
      *
-     * @throws \InvalidArgumentException If the relation with the given name does not exist
+     * @throws InvalidArgumentException If the relation with the given name does not exist
      */
-    public function get($name)
+    public function get(string $name): Relation
     {
         $this->assertRelationExists($name);
 
@@ -57,9 +58,9 @@ public function get($name)
      *
      * @return $this
      *
-     * @throws \InvalidArgumentException If a relation with the given name already exists
+     * @throws InvalidArgumentException If a relation with the given name already exists
      */
-    public function add(Relation $relation)
+    public function add(Relation $relation): static
     {
         $name = $relation->getName();
 
@@ -79,14 +80,14 @@ public function add(Relation $relation)
      *
      * @return BelongsTo|BelongsToOne|BelongsToMany|HasMany|HasOne|Relation
      *
-     * @throws \InvalidArgumentException If the target model class is not of type string
+     * @throws InvalidArgumentException If the target model class is not of type string
      */
-    public function create($class, $name, $targetClass)
+    public function create(string $class, string $name, string $targetClass): Relation
     {
         $relation = new $class();
 
         if (! $relation instanceof Relation) {
-            throw new \InvalidArgumentException(sprintf(
+            throw new InvalidArgumentException(sprintf(
                 '%s() expects parameter 1 to be a subclass of %s, %s given',
                 __METHOD__,
                 Relation::class,
@@ -97,7 +98,7 @@ public function create($class, $name, $targetClass)
         // Test target model
         $target = new $targetClass();
         if (! $target instanceof Model) {
-            throw new \InvalidArgumentException(sprintf(
+            throw new InvalidArgumentException(sprintf(
                 '%s() expects parameter 3 to be a subclass of %s, %s given',
                 __METHOD__,
                 Model::class,
@@ -122,7 +123,7 @@ public function create($class, $name, $targetClass)
      *
      * @return HasOne
      */
-    public function hasOne($name, $targetClass)
+    public function hasOne(string $name, string $targetClass): HasOne
     {
         /** @var HasOne $relation */
         $relation = $this->create(HasOne::class, $name, $targetClass);
@@ -140,7 +141,7 @@ public function hasOne($name, $targetClass)
      *
      * @return HasMany
      */
-    public function hasMany($name, $targetClass)
+    public function hasMany(string $name, string $targetClass): HasMany
     {
         /** @var HasMany $relation */
         $relation = $this->create(HasMany::class, $name, $targetClass);
@@ -158,7 +159,7 @@ public function hasMany($name, $targetClass)
      *
      * @return BelongsTo
      */
-    public function belongsTo($name, $targetClass)
+    public function belongsTo(string $name, string $targetClass): BelongsTo
     {
         /** @var BelongsTo $relation */
         $relation = $this->create(BelongsTo::class, $name, $targetClass);
@@ -194,7 +195,7 @@ public function belongsToOne(string $name, string $targetClass): BelongsToOne
      *
      * @return BelongsToMany
      */
-    public function belongsToMany($name, $targetClass)
+    public function belongsToMany(string $name, string $targetClass): BelongsToMany
     {
         /** @var BelongsToMany $relation */
         $relation = $this->create(BelongsToMany::class, $name, $targetClass);
@@ -213,11 +214,15 @@ public function getIterator(): Traversable
      * Throw exception if a relation with the given name already exists
      *
      * @param string $name
+     *
+     * @return void
+     *
+     * @throws InvalidArgumentException
      */
-    protected function assertRelationDoesNotYetExist($name)
+    protected function assertRelationDoesNotYetExist(string $name): void
     {
         if ($this->has($name)) {
-            throw new \InvalidArgumentException("Relation '$name' already exists");
+            throw new InvalidArgumentException("Relation '$name' already exists");
         }
     }
 
@@ -225,8 +230,10 @@ protected function assertRelationDoesNotYetExist($name)
      * Throw exception if a relation with the given name does not exist
      *
      * @param string $name
+     *
+     * @throws InvalidArgumentException
      */
-    protected function assertRelationExists($name)
+    protected function assertRelationExists(string $name): void
     {
         if (! $this->has($name)) {
             throw new InvalidRelationException($name);
diff --git a/src/ResolvedExpression.php b/src/ResolvedExpression.php
index 86883da..c1a2347 100644
--- a/src/ResolvedExpression.php
+++ b/src/ResolvedExpression.php
@@ -10,7 +10,7 @@
 class ResolvedExpression extends Expression
 {
     /** @var Generator */
-    protected $resolvedColumns;
+    protected Generator $resolvedColumns;
 
     /**
      * Create a resolved database expression
@@ -28,7 +28,7 @@ public function __construct(ExpressionInterface $expr, Generator $resolvedColumn
     /**
      * @throws RuntimeException In case the columns are not qualified yet
      */
-    public function getColumns()
+    public function getColumns(): array
     {
         if ($this->resolvedColumns->valid()) {
             throw new RuntimeException('Columns are not yet qualified');
@@ -42,7 +42,7 @@ public function getColumns()
      *
      * @return Generator
      */
-    public function getResolvedColumns()
+    public function getResolvedColumns(): Generator
     {
         return $this->resolvedColumns;
     }
diff --git a/src/Resolver.php b/src/Resolver.php
index a055bfe..e054e48 100644
--- a/src/Resolver.php
+++ b/src/Resolver.php
@@ -10,48 +10,45 @@
 use ipl\Orm\Exception\InvalidColumnException;
 use ipl\Orm\Exception\InvalidRelationException;
 use ipl\Orm\Relation\BelongsToMany;
-use ipl\Orm\Relation\BelongsToOne;
 use ipl\Sql\ExpressionInterface;
 use LogicException;
 use OutOfBoundsException;
 use SplObjectStorage;
 
-use function ipl\Stdlib\get_php_type;
-
 /**
  * Column and relation resolver. Acts as glue between queries and models
  */
 class Resolver
 {
     /** @var Query The query to resolve */
-    protected $query;
+    protected Query $query;
 
     /** @var SplObjectStorage Model relations */
-    protected $relations;
+    protected SplObjectStorage $relations;
 
     /** @var SplObjectStorage Model behaviors */
-    protected $behaviors;
+    protected SplObjectStorage $behaviors;
 
     /** @var SplObjectStorage Model defaults */
-    protected $defaults;
+    protected SplObjectStorage $defaults;
 
     /** @var SplObjectStorage Model aliases */
-    protected $aliases;
+    protected SplObjectStorage $aliases;
 
-    /** @var string The alias prefix to use */
-    protected $aliasPrefix;
+    /** @var ?string The alias prefix to use */
+    protected ?string $aliasPrefix = null;
 
     /** @var SplObjectStorage Selectable columns from resolved models */
-    protected $selectableColumns;
+    protected SplObjectStorage $selectableColumns;
 
     /** @var SplObjectStorage Select columns from resolved models */
-    protected $selectColumns;
+    protected SplObjectStorage $selectColumns;
 
     /** @var SplObjectStorage Meta data from models and their direct relations */
-    protected $metaData;
+    protected SplObjectStorage $metaData;
 
     /** @var SplObjectStorage Resolved relations */
-    protected $resolvedRelations;
+    protected SplObjectStorage $resolvedRelations;
 
     /**
      * Create a new resolver
@@ -79,7 +76,7 @@ public function __construct(Query $query)
      *
      * @return Relations
      */
-    public function getRelations(Model $model)
+    public function getRelations(Model $model): Relations
     {
         if (! $this->relations->offsetExists($model)) {
             $relations = new Relations();
@@ -97,7 +94,7 @@ public function getRelations(Model $model)
      *
      * @return Behaviors
      */
-    public function getBehaviors(Model $model)
+    public function getBehaviors(Model $model): Behaviors
     {
         if (! $this->behaviors->offsetExists($model)) {
             $behaviors = new Behaviors();
@@ -141,7 +138,7 @@ public function getDefaults(Model $model): Defaults
      *
      * @throws OutOfBoundsException If no alias exists for the given model
      */
-    public function getAlias(Model $model)
+    public function getAlias(Model $model): string
     {
         if (! $this->aliases->offsetExists($model)) {
             throw new OutOfBoundsException(sprintf(
@@ -161,7 +158,7 @@ public function getAlias(Model $model)
      *
      * @return $this
      */
-    public function setAlias(Model $model, $alias)
+    public function setAlias(Model $model, string $alias): static
     {
         $this->aliases[$model] = $alias;
 
@@ -171,9 +168,9 @@ public function setAlias(Model $model, $alias)
     /**
      * Get the alias prefix
      *
-     * @return string
+     * @return ?string
      */
-    public function getAliasPrefix()
+    public function getAliasPrefix(): ?string
     {
         return $this->aliasPrefix;
     }
@@ -185,7 +182,7 @@ public function getAliasPrefix()
      *
      * @return $this
      */
-    public function setAliasPrefix($alias)
+    public function setAliasPrefix(string $alias): static
     {
         $this->aliasPrefix = $alias;
 
@@ -200,7 +197,7 @@ public function setAliasPrefix($alias)
      *
      * @return bool
      */
-    public function hasSelectableColumn(Model $subject, $column)
+    public function hasSelectableColumn(Model $subject, string $column): bool
     {
         if (! $this->selectableColumns->offsetExists($subject)) {
             $this->collectColumns($subject);
@@ -221,7 +218,7 @@ public function hasSelectableColumn(Model $subject, $column)
      *
      * @return array
      */
-    public function getSelectableColumns(Model $subject)
+    public function getSelectableColumns(Model $subject): array
     {
         if (! $this->selectableColumns->offsetExists($subject)) {
             $this->collectColumns($subject);
@@ -237,7 +234,7 @@ public function getSelectableColumns(Model $subject)
      *
      * @return array Select columns suitable for {@link \ipl\Sql\Select::columns()}
      */
-    public function getSelectColumns(Model $subject)
+    public function getSelectColumns(Model $subject): array
     {
         if (! $this->selectColumns->offsetExists($subject)) {
             $this->collectColumns($subject);
@@ -253,7 +250,7 @@ public function getSelectColumns(Model $subject)
      *
      * @return array Column paths as keys (relative to $subject) and their meta data as values
      */
-    public function getColumnDefinitions(Model $subject)
+    public function getColumnDefinitions(Model $subject): array
     {
         if (! $this->metaData->offsetExists($subject)) {
             $this->metaData->offsetSet($subject, $this->collectMetaData($subject));
@@ -307,7 +304,7 @@ public function getColumnDefinition(string $columnPath): ColumnDefinition
      *
      * @return string
      */
-    public function qualifyColumnAlias($alias, $tableName)
+    public function qualifyColumnAlias(string $alias, string $tableName): string
     {
         return $tableName . '_' . $alias;
     }
@@ -320,7 +317,7 @@ public function qualifyColumnAlias($alias, $tableName)
      *
      * @return string
      */
-    public function qualifyColumn($column, $tableName)
+    public function qualifyColumn(string $column, string $tableName): string
     {
         return $tableName . '.' . $column;
     }
@@ -333,25 +330,18 @@ public function qualifyColumn($column, $tableName)
      *
      * @return array
      *
-     * @throws InvalidArgumentException If $columns is not iterable
      * @throws InvalidArgumentException If $model is not passed and $columns is not a generator
      */
-    public function qualifyColumns($columns, ?Model $model = null)
+    public function qualifyColumns(iterable $columns, ?Model $model = null): array
     {
         $target = $model ?: $this->query->getModel();
         $targetAlias = $this->getAlias($target);
 
-        if (! is_iterable($columns)) {
-            throw new InvalidArgumentException(
-                sprintf('$columns is not iterable, got %s instead', get_php_type($columns))
-            );
-        }
-
         $qualified = [];
         foreach ($columns as $alias => $column) {
             if (is_int($alias) && is_array($column)) {
                 // $columns is $this->requireAndResolveColumns()
-                list($target, $alias, $columnName) = $column;
+                [$target, $alias, $columnName] = $column;
                 $targetAlias = $this->getAlias($target);
 
                 // Thanks to PHP 5.6 where `list` is evaluated from right to left. It will extract
@@ -387,25 +377,18 @@ public function qualifyColumns($columns, ?Model $model = null)
      *
      * @return array
      *
-     * @throws InvalidArgumentException If $columns is not iterable
      * @throws InvalidArgumentException If $model is not passed and $columns is not a generator
      */
-    public function qualifyColumnsAndAliases($columns, ?Model $model = null, $autoAlias = true)
+    public function qualifyColumnsAndAliases(iterable $columns, ?Model $model = null, bool $autoAlias = true): array
     {
         $target = $model ?: $this->query->getModel();
         $targetAlias = $this->getAlias($target);
 
-        if (! is_iterable($columns)) {
-            throw new InvalidArgumentException(
-                sprintf('$columns is not iterable, got %s instead', get_php_type($columns))
-            );
-        }
-
         $qualified = [];
         foreach ($columns as $alias => $column) {
             if (is_int($alias) && is_array($column)) {
                 // $columns is $this->requireAndResolveColumns()
-                list($target, $alias, $columnName) = $column;
+                [$target, $alias, $columnName] = $column;
                 $targetAlias = $this->getAlias($target);
 
                 // Thanks to PHP 5.6 where `list` is evaluated from right to left. It will extract
@@ -455,7 +438,7 @@ public function qualifyColumnsAndAliases($columns, ?Model $model = null, $autoAl
      *
      * @return string
      */
-    public function qualifyPath($path, $tableName)
+    public function qualifyPath(string $path, string $tableName): string
     {
         $segments = explode('.', $path, 2);
 
@@ -475,7 +458,7 @@ public function qualifyPath($path, $tableName)
      *
      * @return bool
      */
-    public function isDistinctRelation($path)
+    public function isDistinctRelation(string $path): bool
     {
         foreach ($this->resolveRelations($path) as $relation) {
             if (! $relation->isOne()) {
@@ -496,7 +479,7 @@ public function isDistinctRelation($path)
      *
      * @return Relation
      */
-    public function resolveRelation($path, ?Model $subject = null)
+    public function resolveRelation(string $path, ?Model $subject = null): Relation
     {
         $subject = $subject ?: $this->query->getModel();
         if (! $this->resolvedRelations->offsetExists($subject) || ! isset($this->resolvedRelations[$subject][$path])) {
@@ -520,7 +503,7 @@ public function resolveRelation($path, ?Model $subject = null)
      * @throws InvalidArgumentException In case $path is not fully qualified
      * @throws InvalidRelationException In case a relation is unknown
      */
-    public function resolveRelations($path, ?Model $subject = null)
+    public function resolveRelations(string $path, ?Model $subject = null): Generator
     {
         $relations = explode('.', $path);
         $subject = $subject ?: $this->query->getModel();
@@ -600,7 +583,7 @@ public function resolveRelations($path, ?Model $subject = null)
      *
      * @throws InvalidColumnException If a column does not exist
      */
-    public function requireAndResolveColumns(array $columns, ?Model $model = null)
+    public function requireAndResolveColumns(array $columns, ?Model $model = null): Generator
     {
         $model = $model ?: $this->query->getModel();
         $tableName = $model->getTableAlias();
@@ -739,8 +722,10 @@ public function requireAndResolveColumns(array $columns, ?Model $model = null)
      * Collect all selectable columns from the given model
      *
      * @param Model $subject
+     *
+     * @return void
      */
-    protected function collectColumns(Model $subject)
+    protected function collectColumns(Model $subject): void
     {
         // Don't fail if Model::getColumns() also contains the primary key columns
         $columns = array_merge((array) $subject->getKeyName(), (array) $subject->getColumns());
@@ -768,8 +753,10 @@ protected function collectColumns(Model $subject)
      * @param Model $subject
      *
      * @return array
+     *
+     * @throws LogicException If a column definition's name differs from its array index
      */
-    protected function collectMetaData(Model $subject)
+    protected function collectMetaData(Model $subject): array
     {
         $definitions = [];
         foreach ($subject->getColumnDefinitions() as $name => $data) {
diff --git a/src/UnionQuery.php b/src/UnionQuery.php
index a6405ce..b1860c1 100644
--- a/src/UnionQuery.php
+++ b/src/UnionQuery.php
@@ -6,22 +6,22 @@
 
 class UnionQuery extends Query
 {
-    /** @var Query[] Underlying queries */
-    private $unions;
+    /** @var ?Query[] Underlying queries */
+    private ?array $unions = null;
 
     /**
      * Get the underlying queries
      *
      * @return Query[]
      */
-    public function getUnions()
+    public function getUnions(): array
     {
         if ($this->unions === null) {
             $this->unions = [];
 
             /** @var UnionModel $model */
             $model = $this->getModel();
-            foreach ($model->getUnions() as list($target, $relations, $columns)) {
+            foreach ($model->getUnions() as [$target, $relations, $columns]) {
                 /** @var class-string<Model> $target */
                 $query = $target::on($this->getDb())
                     ->columns($columns)
@@ -35,7 +35,7 @@ public function getUnions()
         return $this->unions;
     }
 
-    public function getSelectBase()
+    public function getSelectBase(): Select
     {
         if ($this->selectBase === null) {
             $this->selectBase = new Select();
diff --git a/tests/BelongsToManyTest.php b/tests/BelongsToManyTest.php
index 10ebdeb..ce552f4 100644
--- a/tests/BelongsToManyTest.php
+++ b/tests/BelongsToManyTest.php
@@ -39,7 +39,7 @@ public function testResolveDefaultKeys()
             $relations
                 ->get('user')
                 ->setSource($model)
-                ->resolve() as list($from, $to, $keys)
+                ->resolve() as [$from, $to, $keys]
         ) {
             reset($keys);
             $actual[] = [
@@ -76,7 +76,7 @@ public function testResolveRespectsCustomKeysInTroughModels()
             $relations
                 ->get('user_custom_keys')
                 ->setSource($model)
-                ->resolve() as list($from, $to, $keys)
+                ->resolve() as [$from, $to, $keys]
         ) {
             reset($keys);
             $actual[] = [
diff --git a/tests/RelationTest.php b/tests/RelationTest.php
index 6a225c1..1148645 100644
--- a/tests/RelationTest.php
+++ b/tests/RelationTest.php
@@ -2,7 +2,6 @@
 
 namespace ipl\Tests\Orm;
 
-use InvalidArgumentException;
 use ipl\Orm\Relation;
 
 class RelationTest extends \PHPUnit\Framework\TestCase
@@ -81,13 +80,6 @@ public function testGetTargetClassReturnsCorrectTargetClassIfSet()
         $this->assertSame($targetClass, $relation->getTargetClass());
     }
 
-    public function testSetTargetClassThrowsInvalidArgumentExceptionIfNotString()
-    {
-        $this->expectException(InvalidArgumentException::class);
-
-        (new Relation())->setTargetClass(new TestModel());
-    }
-
     public function testGetDefaultCandidateKeyReturnsEmptyArrayIfSourceModelsPrimaryKeyIsUnset()
     {
         $candidateKey = Relation::getDefaultCandidateKey(new TestModel());
diff --git a/tests/ResolverTest.php b/tests/ResolverTest.php
index c982051..a7fbad4 100644
--- a/tests/ResolverTest.php
+++ b/tests/ResolverTest.php
@@ -3,7 +3,6 @@
 namespace ipl\Tests\Orm;
 
 use ipl\Orm\Query;
-use ipl\Orm\Resolver;
 use ipl\Sql\Expression;
 use ipl\Sql\QueryBuilder;
 use PHPUnit\Framework\TestCase;
diff --git a/tests/SqlTest.php b/tests/SqlTest.php
index ef3f2a8..372429c 100644
--- a/tests/SqlTest.php
+++ b/tests/SqlTest.php
@@ -382,7 +382,7 @@ public function assertSql($sql, $query, $values = null)
         // Reduce whitespaces to just one space
         $sql = preg_replace('/\s+/', ' ', trim($sql));
 
-        list($stmt, $bind) = $this->queryBuilder->assemble($query);
+        [$stmt, $bind] = $this->queryBuilder->assemble($query);
 
         $this->assertSame($sql, $stmt);