simplify adding relations

orm/fields/foreign_key.py

@@ -1,4 +1,4 @@
-from typing import Type, List, Any, Union, TYPE_CHECKING
+from typing import Type, List, Any, Union, TYPE_CHECKING, Optional
 
 import sqlalchemy
 from pydantic import BaseModel
@@ -12,9 +12,8 @@ if TYPE_CHECKING:  # pragma no cover
 
 
 def create_dummy_instance(fk: Type["Model"], pk: int = None) -> "Model":
-    init_dict = {fk.__pkname__: pk or -1}
     init_dict = {
-        **init_dict,
+        **{fk.__pkname__: pk or -1},
         **{
             k: create_dummy_instance(v.to)
             for k, v in fk.__model_fields__.items()
@@ -52,7 +51,10 @@ class ForeignKey(BaseField):
 
     def expand_relationship(
             self, value: Any, child: "Model"
-    ) -> Union["Model", List["Model"]]:
+    ) -> Optional[Union["Model", List["Model"]]]:
+
+        if value is None:
+            return None
 
         if isinstance(value, orm.models.Model) and not isinstance(value, self.to):
             raise RelationshipInstanceError(
@@ -77,15 +79,10 @@ class ForeignKey(BaseField):
                 )
             model = create_dummy_instance(fk=self.to, pk=value)
 
-        self.add_to_relationship_registry(model, child)
-
-        return model
-
-    def add_to_relationship_registry(self, model: "Model", child: "Model") -> None:
         model._orm_relationship_manager.add_relation(
-            model.__class__.__name__.lower(),
-            child.__class__.__name__.lower(),
             model,
             child,
             virtual=self.virtual,
         )
+
+        return model

orm/relations.py

@@ -57,14 +57,14 @@ class RelationshipManager:
 
     def add_relation(
             self,
-        parent_name: str,
-        child_name: str,
             parent: "FakePydantic",
             child: "FakePydantic",
             virtual: bool = False,
     ) -> None:
         parent_id = parent._orm_id
         child_id = child._orm_id
+        parent_name = parent.get_name()
+        child_name = child.get_name()
         if virtual:
             child_name, parent_name = parent_name, child_name
             child_id, parent_id = parent_id, child_id

tests/test_foreign_keys.py

@@ -74,6 +74,13 @@ async def test_wrong_query_foreign_key_type():
         Track(title="The Error", album="wrong_pk_type")
 
 
+@pytest.mark.asyncio
+async def test_setting_explicitly_empty_relation():
+    async with database:
+        track = Track(album=None, title="The Bird", position=1)
+        assert track.album is None
+
+
 @pytest.mark.asyncio
 async def test_model_crud():
     async with database: