[kotlin-client] Add oneOf discriminator support for multiplatform

bin/configs/kotlin-multiplatform-oneOf-discriminator.yaml

@@ -0,0 +1,9 @@
+generatorName: kotlin
+outputDir: samples/client/petstore/kotlin-multiplatform-oneOf-discriminator
+library: multiplatform
+inputSpec: modules/openapi-generator/src/test/resources/3_0/kotlin/polymorphism-oneof-discriminator-simple.yaml
+templateDir: modules/openapi-generator/src/main/resources/kotlin-client
+additionalProperties:
+  artifactId: kotlin-multiplatform-oneOf-discriminator
+  dateLibrary: kotlinx-datetime
+  useOneOfDiscriminatorSealedClass: true

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinClientCodegen.java

@@ -108,6 +108,8 @@ public class KotlinClientCodegen extends AbstractKotlinCodegen {
 
     public static final String COMPANION_OBJECT = "companionObject";
 
+    public static final String USE_ONEOF_DISCRIMINATOR_SEALED_CLASS = "useOneOfDiscriminatorSealedClass";
+
     protected static final String VENDOR_EXTENSION_BASE_NAME_LITERAL = "x-base-name-literal";
 
 
@@ -126,6 +128,7 @@ public class KotlinClientCodegen extends AbstractKotlinCodegen {
     @Setter protected boolean generateOneOfAnyOfWrappers = true;
     @Getter @Setter protected boolean failOnUnknownProperties = false;
     @Setter protected boolean companionObject = false;
+    @Setter protected boolean useOneOfDiscriminatorSealedClass = false;
 
     protected String authFolder;
 
@@ -293,6 +296,8 @@ public KotlinClientCodegen() {
 
         cliOptions.add(CliOption.newBoolean(COMPANION_OBJECT, "Whether to generate companion objects in data classes, enabling companion extensions.", false));
 
+        cliOptions.add(CliOption.newBoolean(USE_ONEOF_DISCRIMINATOR_SEALED_CLASS, "When true, oneOf schemas with discriminators generate sealed class hierarchies instead of brute-force wrapper classes. Only applies to the multiplatform library.", false));
+
         CliOption serializationLibraryOpt = new CliOption(CodegenConstants.SERIALIZATION_LIBRARY, SERIALIZATION_LIBRARY_DESC);
         cliOptions.add(serializationLibraryOpt.defaultValue(serializationLibrary.name()));
 
@@ -494,6 +499,10 @@ public void processOpts() {
             setGenerateOneOfAnyOfWrappers(convertPropertyToBooleanAndWriteBack(GENERATE_ONEOF_ANYOF_WRAPPERS));
         }
 
+        if (additionalProperties.containsKey(USE_ONEOF_DISCRIMINATOR_SEALED_CLASS)) {
+            setUseOneOfDiscriminatorSealedClass(convertPropertyToBooleanAndWriteBack(USE_ONEOF_DISCRIMINATOR_SEALED_CLASS));
+        }
+
         if (additionalProperties.containsKey(FAIL_ON_UNKNOWN_PROPERTIES)) {
             setFailOnUnknownProperties(convertPropertyToBooleanAndWriteBack(FAIL_ON_UNKNOWN_PROPERTIES));
         } else {
@@ -997,10 +1006,53 @@ public Map<String, ModelsMap> postProcessAllModels(Map<String, ModelsMap> objs)
                         continue;
                     }
 
+                    boolean isOneOfModel = cm.oneOf != null && !cm.oneOf.isEmpty();
+
+                    // For multiplatform oneOf with discriminator, generate sealed class polymorphism
+                    // instead of brute-force wrapper deserialization (opt-in via useOneOfDiscriminatorSealedClass)
+                    if (isOneOfModel && useOneOfDiscriminatorSealedClass && getLibrary() != null && getLibrary().equals(MULTIPLATFORM)) {
+                        // Remove discriminator property from the parent - kotlinx.serialization handles it via @JsonClassDiscriminator
+                        getAllVarProperties(cm).forEach(list -> list.removeIf(var -> var.name.equals(discriminator.getPropertyName())));
+
+                        // Clear all merged properties from oneOf parent - they belong to children only
+                        cm.vars.clear();
+                        cm.allVars.clear();
+                        cm.requiredVars.clear();
+                        cm.optionalVars.clear();
+                        cm.setHasVars(false);
+
+                        // Mark this model to use sealed class rendering in the oneOf template
+                        cm.vendorExtensions.put("x-oneof-sealed-class", true);
+
+                        for (CodegenDiscriminator.MappedModel mappedModel : discriminator.getMappedModels()) {
+                            CodegenModel childModel = mappedModel.getModel();
+
+                            // Set parent-child relationship
+                            childModel.setParent(cm.getClassname());
+                            childModel.setParentModel(cm);
+
+                            // Set discriminator value for @SerialName annotation
+                            CodegenProperty additionalProperties = childModel.getAdditionalProperties();
+                            if (additionalProperties == null) {
+                                additionalProperties = new CodegenProperty();
+                                childModel.setAdditionalProperties(additionalProperties);
+                            }
+                            additionalProperties.discriminatorValue = mappedModel.getMappingName();
+
+                            // Remove discriminator property from child - handled by kotlinx.serialization
+                            getAllVarProperties(childModel).forEach(list -> list.removeIf(prop -> prop.name.equals(discriminator.getPropertyName())));
+
+                            if (childModel.vars.isEmpty() && !childModel.isEnum && !childModel.isAlias) {
+                                childModel.setHasVars(false);
+                            }
+                        }
+                        continue;
+                    }
+
                     // When using generateOneOfAnyOfWrappers and encountering oneOf, we keep discriminator properties,
                     // because single entity can be referenced in multiple "parent" entities,
                     // so discriminator for one might not be discriminator for another.
-                    boolean shouldKeepDiscriminatorField = generateOneOfAnyOfWrappers && cm.oneOf != null && !cm.oneOf.isEmpty();
+                    boolean shouldKeepDiscriminatorField = generateOneOfAnyOfWrappers && isOneOfModel;
 
                     if (shouldKeepDiscriminatorField) {
                         continue;

modules/openapi-generator/src/main/resources/kotlin-client/libraries/multiplatform/oneof_class.mustache

@@ -1,4 +1,23 @@
 {{#multiplatform}}
+{{#vendorExtensions.x-oneof-sealed-class}}
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.json.JsonClassDiscriminator
+
+/**
+ * {{{description}}}
+ *
+ */
+{{#isDeprecated}}
+@Deprecated(message = "This schema is deprecated.")
+{{/isDeprecated}}
+@Serializable
+@OptIn(ExperimentalSerializationApi::class)
+@JsonClassDiscriminator(discriminator = "{{{discriminator.propertyName}}}")
+{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}sealed class {{classname}}
+{{/vendorExtensions.x-oneof-sealed-class}}
+{{^vendorExtensions.x-oneof-sealed-class}}
 import kotlinx.serialization.*
 import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
@@ -76,4 +95,5 @@ import kotlinx.serialization.json.*
         }
     }
 }
-{{/multiplatform}}
+{{/vendorExtensions.x-oneof-sealed-class}}
+{{/multiplatform}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/kotlin/KotlinClientCodegenModelTest.java

@@ -923,6 +923,56 @@ public void testCompanionObjectGeneratesCompanionInModel() throws IOException {
         TestUtils.assertFileContains(petModel, "companion object { }");
     }
 
+    @Test(description = "generate multiplatform oneOf with discriminator as sealed class")
+    public void multiplatformOneOfDiscriminatorSealedClass() throws IOException {
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("kotlin")
+                .setLibrary("multiplatform")
+                .setAdditionalProperties(new HashMap<>() {{
+                    put("dateLibrary", "kotlinx-datetime");
+                    put("useOneOfDiscriminatorSealedClass", true);
+                }})
+                .setInputSpec("src/test/resources/3_0/kotlin/polymorphism-oneof-discriminator-simple.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        final ClientOptInput clientOptInput = configurator.toClientOptInput();
+        DefaultGenerator generator = new DefaultGenerator();
+        List<File> files = generator.opts(clientOptInput).generate();
+
+        final Path petKt = Paths.get(output + "/src/commonMain/kotlin/org/openapitools/client/models/Pet.kt");
+        // parent is sealed class with discriminator annotation
+        TestUtils.assertFileContains(petKt, "sealed class Pet");
+        TestUtils.assertFileContains(petKt, "@JsonClassDiscriminator(discriminator = \"petType\")");
+        TestUtils.assertFileContains(petKt, "@Serializable");
+        // parent does not contain discriminator property or child properties
+        TestUtils.assertFileNotContains(petKt, "val petType");
+        TestUtils.assertFileNotContains(petKt, "val breed");
+        TestUtils.assertFileNotContains(petKt, "val color");
+
+        final Path dogKt = Paths.get(output + "/src/commonMain/kotlin/org/openapitools/client/models/Dog.kt");
+        // child extends parent
+        TestUtils.assertFileContains(dogKt, "data class Dog");
+        TestUtils.assertFileContains(dogKt, ": Pet()");
+        // child has discriminator value annotation
+        TestUtils.assertFileContains(dogKt, "@SerialName(value = \"DOG\")");
+        // child does not contain discriminator property
+        TestUtils.assertFileNotContains(dogKt, "val petType");
+        // child has its own properties
+        TestUtils.assertFileContains(dogKt, "val breed");
+
+        final Path catKt = Paths.get(output + "/src/commonMain/kotlin/org/openapitools/client/models/Cat.kt");
+        // child extends parent
+        TestUtils.assertFileContains(catKt, "data class Cat");
+        TestUtils.assertFileContains(catKt, ": Pet()");
+        // child has discriminator value annotation
+        TestUtils.assertFileContains(catKt, "@SerialName(value = \"CAT\")");
+        // child has its own properties
+        TestUtils.assertFileContains(catKt, "val color");
+    }
+
     private static class ModelNameTest {
         private final String expectedName;
         private final String expectedClassName;

modules/openapi-generator/src/test/resources/3_0/kotlin/polymorphism-oneof-discriminator-simple.yaml

@@ -0,0 +1,68 @@
+openapi: 3.0.1
+info:
+  title: Example
+  description: OneOf with discriminator example
+  version: '0.1'
+  contact:
+    email: contact@example.org
+    url: 'https://example.org'
+servers:
+  - url: http://example.org
+tags:
+  - name: pet
+paths:
+  '/v1/pet/{id}':
+    get:
+      tags:
+        - pet
+      responses:
+        '200':
+          description: OK
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/pet'
+      operationId: get-pet
+    parameters:
+      - schema:
+          type: string
+          format: uuid
+        name: id
+        in: path
+        required: true
+components:
+  schemas:
+    pet:
+      title: A pet
+      oneOf:
+        - $ref: '#/components/schemas/dog'
+        - $ref: '#/components/schemas/cat'
+      discriminator:
+        propertyName: petType
+        mapping:
+          DOG: '#/components/schemas/dog'
+          CAT: '#/components/schemas/cat'
+    dog:
+      title: A dog
+      required:
+        - petType
+        - breed
+      properties:
+        petType:
+          type: string
+        breed:
+          type: string
+        bark:
+          type: boolean
+    cat:
+      title: A cat
+      required:
+        - petType
+        - color
+      properties:
+        petType:
+          type: string
+        color:
+          type: string
+        indoor:
+          type: boolean

samples/client/petstore/kotlin-multiplatform-oneOf-discriminator/.openapi-generator-ignore

@@ -0,0 +1,23 @@
+# OpenAPI Generator Ignore
+# Generated by openapi-generator https://github.com/openapitools/openapi-generator
+
+# Use this file to prevent files from being overwritten by the generator.
+# The patterns follow closely to .gitignore or .dockerignore.
+
+# As an example, the C# client generator defines ApiClient.cs.
+# You can make changes and tell OpenAPI Generator to ignore just this file by uncommenting the following line:
+#ApiClient.cs
+
+# You can match any string of characters against a directory, file or extension with a single asterisk (*):
+#foo/*/qux
+# The above matches foo/bar/qux and foo/baz/qux, but not foo/bar/baz/qux
+
+# You can recursively match patterns against a directory, file or extension with a double asterisk (**):
+#foo/**/qux
+# This matches foo/bar/qux, foo/baz/qux, and foo/bar/baz/qux
+
+# You can also negate patterns with an exclamation (!).
+# For example, you can ignore all files in a docs folder with the file extension .md:
+#docs/*.md
+# Then explicitly reverse the ignore rule for a single file:
+#!docs/README.md

samples/client/petstore/kotlin-multiplatform-oneOf-discriminator/.openapi-generator/FILES

@@ -0,0 +1,33 @@
+.openapi-generator-ignore
+README.md
+build.gradle.kts
+docs/Cat.md
+docs/Dog.md
+docs/Pet.md
+docs/PetApi.md
+gradle/wrapper/gradle-wrapper.jar
+gradle/wrapper/gradle-wrapper.properties
+gradlew
+gradlew.bat
+settings.gradle.kts
+src/commonMain/kotlin/org/openapitools/client/apis/PetApi.kt
+src/commonMain/kotlin/org/openapitools/client/auth/ApiKeyAuth.kt
+src/commonMain/kotlin/org/openapitools/client/auth/Authentication.kt
+src/commonMain/kotlin/org/openapitools/client/auth/HttpBasicAuth.kt
+src/commonMain/kotlin/org/openapitools/client/auth/HttpBearerAuth.kt
+src/commonMain/kotlin/org/openapitools/client/auth/OAuth.kt
+src/commonMain/kotlin/org/openapitools/client/infrastructure/ApiAbstractions.kt
+src/commonMain/kotlin/org/openapitools/client/infrastructure/ApiClient.kt
+src/commonMain/kotlin/org/openapitools/client/infrastructure/Base64ByteArray.kt
+src/commonMain/kotlin/org/openapitools/client/infrastructure/HttpResponse.kt
+src/commonMain/kotlin/org/openapitools/client/infrastructure/OctetByteArray.kt
+src/commonMain/kotlin/org/openapitools/client/infrastructure/PartConfig.kt
+src/commonMain/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt
+src/commonMain/kotlin/org/openapitools/client/infrastructure/RequestMethod.kt
+src/commonMain/kotlin/org/openapitools/client/models/Cat.kt
+src/commonMain/kotlin/org/openapitools/client/models/Dog.kt
+src/commonMain/kotlin/org/openapitools/client/models/Pet.kt
+src/test/kotlin/org/openapitools/client/apis/PetApiTest.kt
+src/test/kotlin/org/openapitools/client/models/CatTest.kt
+src/test/kotlin/org/openapitools/client/models/DogTest.kt
+src/test/kotlin/org/openapitools/client/models/PetTest.kt

samples/client/petstore/kotlin-multiplatform-oneOf-discriminator/.openapi-generator/VERSION

@@ -0,0 +1 @@
+7.21.0-SNAPSHOT

samples/client/petstore/kotlin-multiplatform-oneOf-discriminator/README.md

@@ -0,0 +1,60 @@
+# org.openapitools.client - Kotlin client library for Example
+
+OneOf with discriminator example
+
+## Overview
+This API client was generated by the [OpenAPI Generator](https://openapi-generator.tech) project.  By using the [openapi-spec](https://github.com/OAI/OpenAPI-Specification) from a remote server, you can easily generate an API client.
+
+- API version: 0.1
+- Package version: 
+- Generator version: 7.21.0-SNAPSHOT
+- Build package: org.openapitools.codegen.languages.KotlinClientCodegen
+For more information, please visit [https://example.org](https://example.org)
+
+## Requires
+
+* Kotlin 2.2.20
+
+## Build
+
+```
+./gradlew check assemble
+```
+
+This runs all tests and packages the library.
+
+## Features/Implementation Notes
+
+* Supports JSON inputs/outputs, File inputs, and Form inputs.
+* Supports collection formats for query parameters: csv, tsv, ssv, pipes.
+* Some Kotlin and Java types are fully qualified to avoid conflicts with types defined in OpenAPI definitions.
+
+
+<a id="documentation-for-api-endpoints"></a>
+## Documentation for API Endpoints
+
+All URIs are relative to *http://example.org*
+
+| Class | Method | HTTP request | Description |
+| ------------ | ------------- | ------------- | ------------- |
+| *PetApi* | [**getPet**](docs/PetApi.md#getpet) | **GET** /v1/pet/{id} |  |
+
+
+<a id="documentation-for-models"></a>
+## Documentation for Models
+
+ - [org.openapitools.client.models.Cat](docs/Cat.md)
+ - [org.openapitools.client.models.Dog](docs/Dog.md)
+ - [org.openapitools.client.models.Pet](docs/Pet.md)
+
+
+<a id="documentation-for-authorization"></a>
+## Documentation for Authorization
+
+Endpoints do not require authorization.
+
+
+
+## Author
+
+contact@example.org