Fix inc/dec rejecting deltas outside a field's min_value/max_value

AUTHORS

@@ -266,3 +266,4 @@ that much better:
  * Terence Honles (https://github.com/terencehonles)
  * Sean Bermejo (https://github.com/seanbermejo)
  * Juan Gutierrez (https://github.com/juannyg)
+ * DSeaStar (https://github.com/DSeaStar)

docs/changelog.rst

@@ -7,6 +7,7 @@ Changelog
 Development
 ===========
 - (Fill this out as you fix issues and develop your features).
+- Fix inc/dec atomic updates rejecting deltas outside a field's min_value/max_value #2339
 - Add a warning that ``mongoengine.org`` is no longer controlled by the MongoEngine
   project and appears to be an expired domain takeover.
 - Fix querying GenericReferenceField with __in operator #2886

mongoengine/base/fields.py

@@ -236,7 +236,10 @@ def _to_mongo_safe_call(self, value, use_db_field=True, fields=None):
 
     def prepare_query_value(self, op, value):
         """Prepare a value that is being used in a query for PyMongo."""
-        if op in UPDATE_OPERATORS:
+        # $inc/$dec apply a delta, not the stored value. Checking min_value
+        # / max_value against the delta rejects legitimate decrements on
+        # fields with min_value=0 (see #2339).
+        if op in UPDATE_OPERATORS and op not in ("inc", "dec"):
             self.validate(value)
         return value
 

tests/queryset/test_transform.py

@@ -83,6 +83,32 @@ class BlogPost(Document):
         update = transform.update(BlogPost, push_all__tags=["mongo", "db"])
         assert update == {"$push": {"tags": {"$each": ["mongo", "db"]}}}
 
+    def test_transform_update_inc_dec_ignores_min_max(self):
+        """inc/dec pass a delta; min_value/max_value apply to stored values (#2339)."""
+
+        class Account(Document):
+            amount = FloatField(min_value=0, required=True)
+            count = IntField(min_value=0, max_value=100)
+            money = DecimalField(min_value=0)
+
+        update = transform.update(Account, dec__amount=10)
+        assert update == {"$inc": {"amount": -10.0}}
+
+        update = transform.update(Account, inc__count=1)
+        assert update == {"$inc": {"count": 1}}
+
+        update = transform.update(Account, dec__count=5)
+        assert update == {"$inc": {"count": -5}}
+
+        update = transform.update(Account, dec__money=3)
+        assert update == {"$inc": {"money": -3.0}}
+
+        with pytest.raises(ValidationError):
+            transform.update(Account, set__amount=-1)
+
+        with pytest.raises(ValidationError):
+            transform.update(Account, set__count=101)
+
     def test_transform_update_no_operator_default_to_set(self):
         """Ensure the differences in behvaior between 'push' and 'push_all'"""