Skip virtual fields like GenericForeignKey

Virtual fields (concrete=False) like GenericForeignKey don't have database
columns, causing AttributeError when accessing field.attname. Skip them like
reverse relations are skipped.

Fixes #16

Author: craigds

fieldsignals/signals.py

@@ -65,9 +65,17 @@ def connect(  # type: ignore[override]
         def is_reverse_rel(f: Any) -> bool:
             return f.many_to_many or f.one_to_many or isinstance(f, ForeignObjectRel)
 
+        def is_virtual_field(f: Any) -> bool:
+            """Virtual fields like GenericForeignKey don't have database columns."""
+            return not f.concrete
+
         if fields is None:
             resolved_fields = sender._meta.get_fields()
-            resolved_fields = [f for f in resolved_fields if not is_reverse_rel(f)]
+            resolved_fields = [
+                f
+                for f in resolved_fields
+                if not is_reverse_rel(f) and not is_virtual_field(f)
+            ]
         else:
             resolved_fields = [f for f in sender._meta.get_fields() if f.name in set(fields)]
             for f in resolved_fields:
@@ -76,6 +84,11 @@ def is_reverse_rel(f: Any) -> bool:
                         "django-fieldsignals doesn't handle reverse related fields "
                         f"({f.name} is a {f.__class__.__name__})"
                     )
+                if is_virtual_field(f):
+                    raise ValueError(
+                        "django-fieldsignals doesn't handle virtual fields "
+                        f"({f.name} is a {f.__class__.__name__})"
+                    )
 
         if not resolved_fields:
             raise ValueError("fields must be non-empty")
@@ -166,6 +179,9 @@ def get_and_update_changed_fields(
         deferred_fields = instance.get_deferred_fields()
 
         for field in fields:
+            if not field.concrete:
+                # Skip virtual fields (e.g. GenericForeignKey)
+                continue
             if field.attname in deferred_fields:
                 continue
             # using value_from_object instead of getattr() means we don't traverse foreignkeys

fieldsignals/tests/test_signals.py

@@ -52,6 +52,7 @@ def __init__(self, name: str, m2m: bool = False) -> None:
         self.attname = name
         self.many_to_many = m2m
         self.one_to_many = False
+        self.concrete = True
 
     def value_from_object(self, instance: Any) -> Any:
         return getattr(instance, self.name)
@@ -144,6 +145,29 @@ def get_fields() -> list[Field | MockOneToOneRel]:
             return [Field("f"), MockOneToOneRel("o2o")]
 
 
+class VirtualField:
+    """Mock GenericForeignKey - virtual field with no database column."""
+
+    def __init__(self, name: str) -> None:
+        self.name = name
+        self.many_to_many = False
+        self.one_to_many = False
+        self.concrete = False
+
+
+class FakeModelWithVirtualField:
+    f = "a value"
+    gfk = None
+
+    class _meta:
+        @staticmethod
+        def get_fields() -> list[Field | VirtualField]:
+            return [Field("f"), VirtualField("gfk")]
+
+    def get_deferred_fields(self) -> set[str]:
+        return set()
+
+
 class TestGeneral:
     @pytest.fixture(autouse=True)
     def ready(self) -> None:
@@ -157,9 +181,7 @@ def test_m2m_fields_error(self) -> None:
     def test_one_to_one_rel_field_error(self) -> None:
         with must_be_called(False) as func:
             with pytest.raises(ValueError):
-                post_save_changed.connect(
-                    func, sender=FakeModelWithOneToOne, fields=["o2o", "f"]
-                )
+                post_save_changed.connect(func, sender=FakeModelWithOneToOne, fields=["o2o", "f"])
 
     def test_one_to_one_rel_excluded(self) -> None:
         with must_be_called(False) as func:
@@ -223,6 +245,23 @@ def test_boolean_field_transition_to_valid(self) -> None:
 
         assert func.kwargs["changed_fields"] == {"is_active": (None, True)}
 
+    def test_virtual_field_excluded(self) -> None:
+        """Virtual fields like GenericForeignKey are auto-skipped."""
+        with must_be_called(False) as func:
+            post_save_changed.connect(func, sender=FakeModelWithVirtualField)
+
+            obj = FakeModelWithVirtualField()
+            post_init.send(instance=obj, sender=FakeModelWithVirtualField)
+            post_save.send(instance=obj, sender=FakeModelWithVirtualField)
+
+    def test_virtual_field_error(self) -> None:
+        """Explicitly requesting a virtual field raises clear error."""
+        with must_be_called(False) as func:
+            with pytest.raises(
+                ValueError, match="doesn't handle virtual fields.*gfk.*VirtualField"
+            ):
+                post_save_changed.connect(func, sender=FakeModelWithVirtualField, fields=["gfk"])
+
 
 class TestPostSave:
     @pytest.fixture(autouse=True)