fix: return first PolymorphicParentModelAdmin

src/polymorphic/admin/childadmin.py

@@ -160,26 +160,22 @@ def _get_parent_admin(self):
             # when parent_model is in among child_models, just return super instance
             return super()
 
-        try:
-            return self.admin_site._registry[parent_model]
-        except KeyError:
-            # Admin is not registered for polymorphic_ctype model, but perhaps it's registered
-            # for a intermediate proxy model, between the parent_model and this model.
-            for klass in inspect.getmro(self.model):
-                if not issubclass(klass, parent_model):
-                    continue  # e.g. found a mixin.
-
-                # Fetch admin instance for model class, see if it's a possible candidate.
-                model_admin = self.admin_site._registry.get(klass)
-                if model_admin is not None and isinstance(
-                    model_admin, PolymorphicParentModelAdmin
-                ):
-                    return model_admin  # Success!
-
-            # If we get this far without returning there is no admin available
-            raise ParentAdminNotRegistered(
-                f"No parent admin was registered for a '{parent_model}' model."
-            )
+        # Only PolymorphicParentModelAdmin registrations are considered, whether
+        # they are for the polymorphic root model or an intermediate proxy/model
+        # between the parent_model and this model.
+        for klass in inspect.getmro(self.model):
+            if not issubclass(klass, parent_model):
+                continue  # e.g. found a mixin.
+
+            # Fetch admin instance for model class, see if it's a possible candidate.
+            model_admin = self.admin_site._registry.get(klass)
+            if model_admin is not None and isinstance(model_admin, PolymorphicParentModelAdmin):
+                return model_admin  # Success!
+
+        # If we get this far without returning there is no admin available
+        raise ParentAdminNotRegistered(
+            f"No parent admin was registered for a '{parent_model}' model."
+        )
 
     def response_post_save_add(self, request, obj):
         return self._get_parent_admin().response_post_save_add(request, obj)

src/polymorphic/tests/test_admin.py

@@ -1629,6 +1629,28 @@ class Model2CAdmin(PolymorphicChildModelAdmin):
         parent = c_admin._get_parent_admin()
         assert isinstance(parent, PolymorphicParentModelAdmin)
 
+    def test_child_admin_get_parent_admin_skips_regular_root_admin(self):
+        """_get_parent_admin() ignores a registered root ModelAdmin when finding the polymorphic parent."""
+
+        @self.register(Model2A)
+        class Model2ARootAdmin(admin.ModelAdmin):
+            pass
+
+        @self.register(Model2B)
+        class Model2BParentAdmin(PolymorphicParentModelAdmin):
+            base_model = Model2B
+            child_models = (Model2C,)
+
+        @self.register(Model2C)
+        class Model2CAdmin(PolymorphicChildModelAdmin):
+            base_model = Model2B
+
+        c_admin = self.get_admin_instance(Model2C)
+        parent = c_admin._get_parent_admin()
+
+        assert parent is self.get_admin_instance(Model2B)
+        assert parent is not self.get_admin_instance(Model2A)
+
     def test_child_admin_get_parent_admin_not_registered(self):
         """_get_parent_admin() raises ParentAdminNotRegistered when MRO has no parent admin."""
         from polymorphic.admin.childadmin import ParentAdminNotRegistered