Don't show tags that can cause cyclical dependency. Fixes #1074

.gitignore

@@ -173,7 +173,6 @@ poetry.toml
 .ruff_cache/
 
 # LSP config files
-pyrightconfig.json
 
 # Syncthing
 .stfolder/
@@ -235,7 +234,7 @@ compile_commands.json
 *_qmlcache.qrc
 
 ### VisualStudioCode ###
-.vscode/*
+/.vscode
 # !.vscode/settings.json
 # !.vscode/tasks.json
 # !.vscode/launch.json
@@ -268,3 +267,4 @@ TagStudio.ini
 
 result
 result-*
+uv.lock

src/tagstudio/core/library/alchemy/library.py

@@ -1685,12 +1685,47 @@ def get_tag_hierarchy(self, tag_ids: Iterable[int]) -> dict[int, Tag]:
 
         return all_tags
 
-    def add_parent_tag(self, parent_id: int, child_id: int) -> bool:
+    def get_tag_descendants(self, tag_id: int, session: Session | None = None) -> set[int]:
+        """Return ids of every tag that lists `tag_id` as an ancestor."""
+        owns_session = False
+        if session is None:
+            session = Session(self.engine)
+            owns_session = True
+
+        try:
+            descendants: set[int] = set()
+            frontier: set[int] = {tag_id}
+
+            while frontier:
+                stmt = select(TagParent.child_id).where(TagParent.parent_id.in_(frontier))
+                children = set(session.scalars(stmt).all())
+                children -= descendants
+                children.discard(tag_id)
+                descendants.update(children)
+                frontier = children
+
+            return descendants
+        finally:
+            if owns_session:
+                session.close()
+
+    def _would_create_parent_cycle(self, parent_id: int, child_id: int, session: Session) -> bool:
         if parent_id == child_id:
-            return False
+            return True
+
+        return parent_id in self.get_tag_descendants(child_id, session=session)
 
+    def add_parent_tag(self, parent_id: int, child_id: int) -> bool:
         # open session and save as parent tag
         with Session(self.engine) as session:
+            if self._would_create_parent_cycle(parent_id, child_id, session):
+                logger.warning(
+                    "[Library][add_parent_tag] Prevented cyclical parent assignment",
+                    parent_id=parent_id,
+                    child_id=child_id,
+                )
+                return False
+
             parent_tag = TagParent(
                 parent_id=parent_id,
                 child_id=child_id,
@@ -1814,10 +1849,10 @@ def update_aliases(
             session.add(alias)
 
     def update_parent_tags(self, tag: Tag, parent_ids: list[int] | set[int], session: Session):
-        if tag.id in parent_ids:
-            parent_ids.remove(tag.id)
+        new_parent_ids: set[int] = set(parent_ids)
+        new_parent_ids.discard(tag.id)
 
-        if tag.disambiguation_id not in parent_ids:
+        if tag.disambiguation_id not in new_parent_ids:
             tag.disambiguation_id = None
 
         # load all tag's parent tags to know which to remove
@@ -1826,14 +1861,22 @@ def update_parent_tags(self, tag: Tag, parent_ids: list[int] | set[int], session
         ).all()
 
         for parent_tag in prev_parent_tags:
-            if parent_tag.parent_id not in parent_ids:
+            if parent_tag.parent_id not in new_parent_ids:
                 session.delete(parent_tag)
             else:
                 # no change, remove from list
-                parent_ids.remove(parent_tag.parent_id)
+                new_parent_ids.remove(parent_tag.parent_id)
+
+        # create remaining items
+        for parent_id in list(new_parent_ids):
+            if self._would_create_parent_cycle(parent_id, tag.id, session):
+                logger.warning(
+                    "[Library][update_parent_tags] Prevented cyclical parent assignment",
+                    parent_id=parent_id,
+                    child_id=tag.id,
+                )
+                continue
 
-                # create remaining items
-        for parent_id in parent_ids:
             # add new parent tag
             parent_tag = TagParent(
                 parent_id=parent_id,

src/tagstudio/qt/mixed/build_tag.py

@@ -163,11 +163,12 @@ def __init__(self, library: Library, tag: Tag | None = None) -> None:
         self.parent_tags_add_button.setText("+")
         self.parent_tags_layout.addWidget(self.parent_tags_add_button)
 
-        exclude_ids: list[int] = list()
-        if tag is not None:
-            exclude_ids.append(tag.id)
+        exclude_ids: set[int] = set()
+        if tag is not None and tag.id is not None:
+            exclude_ids.add(tag.id)
+            exclude_ids.update(self.lib.get_tag_descendants(tag.id))
 
-        self.add_tag_modal = TagSearchModal(self.lib, exclude_ids)
+        self.add_tag_modal = TagSearchModal(self.lib, list(exclude_ids))
         self.add_tag_modal.tsp.tag_chosen.connect(lambda x: self.add_parent_tag_callback(x))
         self.parent_tags_add_button.clicked.connect(self.add_tag_modal.show)
 
@@ -562,6 +563,13 @@ def set_tag(self, tag: Tag):
         logger.info("[BuildTagPanel] Setting Tag", tag=tag)
         self.tag = tag
 
+        if tag.id is not None:
+            exclude_ids: set[int] = {tag.id}
+            exclude_ids.update(self.lib.get_tag_descendants(tag.id))
+            self.add_tag_modal.tsp.exclude = list(exclude_ids)
+        else:
+            self.add_tag_modal.tsp.exclude = []
+
         self.name_field.setText(tag.name)
         self.shorthand_field.setText(tag.shorthand or "")
 

src/tagstudio/qt/mixed/field_containers.py

@@ -166,7 +166,7 @@ def get_tag_categories(self, tags: set[Tag]) -> dict[Tag | None, set[Tag]]:
         "Character" -> "Johnny Bravo",
         "TV" -> Johnny Bravo"
         """
-        loop_cutoff = 1024  # Used for stopping the while loop
+        visited_tags: set[int] = set()
 
         hierarchy_tags = self.lib.get_tag_hierarchy(t.id for t in tags)
         categories: dict[Tag | None, set[Tag]] = {None: set()}
@@ -179,16 +179,15 @@ def get_tag_categories(self, tags: set[Tag]) -> dict[Tag | None, set[Tag]]:
             has_category_parent = False
             parent_tags = tag.parent_tags
 
-            loop_counter = 0
-            while len(parent_tags) > 0:
-                # NOTE: This is for preventing infinite loops in the event a tag is parented
-                # to itself cyclically.
-                loop_counter += 1
-                if loop_counter >= loop_cutoff:
-                    break
+            visited_tags.clear()
+            visited_tags.add(tag.id)
 
+            while len(parent_tags) > 0:
                 grandparent_tags: set[Tag] = set()
                 for parent_tag in parent_tags:
+                    if parent_tag.id in visited_tags:
+                        continue
+                    visited_tags.add(parent_tag.id)
                     if parent_tag in categories:
                         categories[parent_tag].add(tag)
                         has_category_parent = True