fix incorrect type serialization when dumping to python

beanie/odm/fields.py

@@ -152,7 +152,7 @@ def __get_pydantic_core_schema__(
                 ),
                 json_schema=str_schema(),
                 serialization=core_schema.plain_serializer_function_ser_schema(
-                    lambda instance: str(instance)
+                    lambda instance: str(instance), when_used="json"
                 ),
             )
 
@@ -361,7 +361,7 @@ async def fetch_many(cls, links: List["Link"]):
         def serialize(value: Union["Link", BaseModel]):
             if isinstance(value, Link):
                 return value.to_dict()
-            return value.model_dump()
+            return value.model_dump(mode="json")
 
         @classmethod
         def build_validation(cls, handler, source_type):
@@ -415,7 +415,8 @@ def __get_pydantic_core_schema__(
                     }
                 ),
                 serialization=core_schema.plain_serializer_function_ser_schema(  # type: ignore
-                    lambda instance: cls.serialize(instance)  # type: ignore
+                    lambda instance: cls.serialize(instance),
+                    when_used="json",  # type: ignore
                 ),
             )
             return core_schema.with_info_plain_validator_function(

tests/odm/test_beanie_object_dumping.py

@@ -0,0 +1,33 @@
+from pydantic import BaseModel, Field
+
+from beanie import Link, PydanticObjectId
+from beanie.odm.utils.pydantic import IS_PYDANTIC_V2
+from tests.odm.models import DocumentTestModelWithSoftDelete
+
+
+class TestModel(BaseModel):
+    my_id: PydanticObjectId = Field(default_factory=PydanticObjectId)
+    fake_doc: Link[DocumentTestModelWithSoftDelete]
+
+
+def data_maker():
+    return TestModel(
+        my_id="5f4e3f3b7c0c9d001f7d4c8e",
+        fake_doc=DocumentTestModelWithSoftDelete(
+            test_int=1, test_str="test", id="5f4e3f3b7c0c9d001f7d4c8f"
+        ),
+    )
+
+
+def test_id_types_preserved_when_dumping_to_python():
+    if IS_PYDANTIC_V2:
+        dumped = data_maker().model_dump(mode="python")
+        assert isinstance(dumped["my_id"], PydanticObjectId)
+        assert isinstance(dumped["fake_doc"]["id"], PydanticObjectId)
+
+
+def test_id_types_serialized_when_dumping_to_json():
+    if IS_PYDANTIC_V2:
+        dumped = data_maker().model_dump(mode="json")
+        assert isinstance(dumped["my_id"], str)
+        assert isinstance(dumped["fake_doc"]["id"], str)