Handle parent tag removal in backend

src/documents/serialisers.py

@@ -1088,22 +1088,27 @@ class DocumentSerializer(
                             doc_id,
                         )
         if "tags" in validated_data:
-            # add all parent tags
+            # Respect tag hierarchy on updates:
-            all_ancestor_tags = set(validated_data["tags"])
+            # - Adding a child adds its ancestors
-            for tag in validated_data["tags"]:
+            # - Removing a parent removes all its descendants
-                all_ancestor_tags.update(tag.get_ancestors())
+            prev_tags = set(instance.tags.all())
-            validated_data["tags"] = list(all_ancestor_tags)
+            requested_tags = set(validated_data["tags"])
-            # remove any children for parents that are being removed
+
-            tag_parents_being_removed = [
+            # Tags being removed in this update and all descendants
-                tag
+            removed_tags = prev_tags - requested_tags
-                for tag in instance.tags.all()
+            blocked_tags = set(removed_tags)
-                if tag not in validated_data["tags"] and tag.get_children_count() > 0
+            for t in removed_tags:
-            ]
+                blocked_tags.update(t.get_descendants())
-            validated_data["tags"] = [
+
-                tag
+            # Add all parent tags
-                for tag in validated_data["tags"]
+            final_tags = set(requested_tags)
-                if tag not in tag_parents_being_removed
+            for t in requested_tags:
-            ]
+                final_tags.update(t.get_ancestors())
+
+            # Drop removed parents and their descendants
+            final_tags.difference_update(blocked_tags)
+
+            validated_data["tags"] = list(final_tags)
         if validated_data.get("remove_inbox_tags"):
             tag_ids_being_added = (
                 [

src/documents/tests/test_tag_hierarchy.py

@@ -51,6 +51,16 @@ class TestTagHierarchy(APITestCase):
         tags = set(self.document.tags.values_list("pk", flat=True))
         assert tags == {self.parent.pk, self.child.pk}
 
+    def test_document_api_remove_parent_removes_children(self):
+        self.document.add_nested_tags([self.parent, self.child])
+        self.client.patch(
+            f"/api/documents/{self.document.pk}/",
+            {"tags": [self.child.pk]},
+            format="json",
+        )
+        self.document.refresh_from_db()
+        assert self.document.tags.count() == 0
+
     def test_document_api_remove_parent_removes_child(self):
         self.document.add_nested_tags([self.child])
         self.client.patch(