Removing relation child while updating nullable fk as None

docs/relations/foreign-key.md

@@ -280,6 +280,26 @@ Finally you can explicitly set it to None (default behavior if no value passed).
 --8<-- "../docs_src/relations/docs001.py"
 ```
 
+You can also clear a `ForeignKey` relation on an already-loaded model by
+assigning `None` — the field must be `nullable` for this to persist. The
+related model is unregistered from the instance immediately, so subsequent
+reads return `None` without requiring a reload:
+
+```python
+track = await Track.objects.select_related("album").get(id=track_id)
+assert track.album is not None
+
+track.album = None
+assert track.album is None  # in-memory state cleared
+
+await track.update(_columns=["album"])  # persists NULL to the database
+```
+
+!!!note
+    Assigning `None` to a non-nullable `ForeignKey` is ignored at the
+    in-memory level. Make the field `nullable=True` (the default) if you
+    need to unassign it.
+
 !!!warning
     In all not None cases the primary key value for related model **has to exist in database**.
 

ormar/models/descriptors/descriptors.py

@@ -1,5 +1,5 @@
 import base64
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, Optional, cast
 
 from ormar.fields.parsers import decode_bytes, encode_json
 
@@ -106,9 +106,19 @@ def __get__(self, instance: "Model", owner: type["Model"]) -> Any:
         return None  # pragma no cover
 
     def __set__(self, instance: "Model", value: Any) -> None:
-        instance.ormar_config.model_fields[self.name].expand_relationship(
-            value=value, child=instance
-        )
+        field = instance.ormar_config.model_fields[self.name]
+        field.expand_relationship(value=value, child=instance)
+
+        if (
+            value is None
+            and field.nullable
+            and not field.virtual
+            and not field.is_multi
+            and self.name in instance._orm
+        ):
+            previous = cast(Optional["Model"], instance._orm.get(self.name))
+            if previous is not None:
+                instance._orm.remove(self.name, previous)
 
         if not isinstance(instance.__dict__.get(self.name), list):
             instance.set_save_status(False)

tests/test_relations/test_foreign_keys.py

@@ -406,3 +406,21 @@ async def test_bulk_update_model_with_children():
                 is_best_seller=True
             ).all()
             assert len(best_seller_albums_db) == 2
+
+
+@pytest.mark.asyncio
+async def test_fk_update_to_none():
+    async with base_ormar_config.database:
+        async with base_ormar_config.database.transaction(force_rollback=True):
+            album = await Album.objects.create(name="Limitless")
+            track = await Track.objects.create(album=album, title="Test1", position=1)
+            assert track.album.name == "Limitless"
+
+            track.album = None
+            assert track.album is None
+
+            updated_track = await track.update(_columns=["album"])
+            assert updated_track.album is None
+
+            reloaded = await Track.objects.get(id=track.id)
+            assert reloaded.album is None