Prevent recursive grouping of albums/tags

How did I not catch this before

constants.py

@@ -98,6 +98,7 @@ ERROR_NO_SUCH_TAG = 'Doesn\'t exist'
 ERROR_NO_TAG_GIVEN = 'No tag name supplied'
 ERROR_SYNONYM_ITSELF = 'Cant apply synonym to itself'
 ERROR_TAG_TOO_SHORT = 'Not enough valid chars'
+ERROR_RECURSIVE_GROUPING = 'Recursive grouping'
 WARNING_MINMAX_INVALID = 'Field "{field}": "{value}" is not a valid request. Ignored.'
 WARNING_MINMAX_OOO = 'Field "{field}": minimum "{min}" maximum "{max}" are out of order. Ignored.'
 WARNING_NO_SUCH_TAG = 'Tag "{tag}" does not exist. Ignored.'

etiquette.py

@@ -685,6 +685,8 @@ def post_edit_tags():
             response = {'error': constants.ERROR_SYNONYM_ITSELF, 'tagname': tag}
         except exceptions.NoSuchTag as e:
             response = {'error': constants.ERROR_NO_SUCH_TAG, 'tagname': tag}
+        except exceptions.RecursiveGrouping as e:
+            response = {'error': constants.ERROR_RECURSIVE_GROUPING, 'tagname': tag}
         except ValueError as e:
             response = {'error': e.args[0], 'tagname': tag}
         else:

exceptions.py

@@ -33,6 +33,9 @@ class UserExists(Exception):
 
 
 # TAG ERRORS
+class RecursiveGrouping(Exception):
+    pass
+
 class TagTooLong(Exception):
     pass
 

objects.py

@@ -43,15 +43,22 @@ class GroupableMixin:
         if not isinstance(member, type(self)):
             raise TypeError('Member must be of type %s' % type(self))
 
+        # Groupables are only allowed to have 1 parent.
+        # Unlike photos which can exist in multiple albums.
         self.photodb.cur.execute('SELECT * FROM tag_group_rel WHERE memberid == ?', [member.id])
         fetch = self.photodb.cur.fetchone()
         if fetch is not None:
-            if fetch[constants.SQL_TAGGROUP['parentid']] == self.id:
+            parent_id = fetch[constants.SQL_TAGGROUP['parentid']]
+            if parent_id == self.id:
                 that_group = self
             else:
-                that_group = self.group_getter(id=fetch[constants.SQL_TAGGROUP['parentid']])
+                that_group = self.group_getter(id=parent_id)
             raise exceptions.GroupExists('%s already in group %s' % (member.name, that_group.name))
 
+        for parent in self.walk_parents():
+            if parent.id == member.id:
+                raise exceptions.RecursiveGrouping('%s is an ancestor of %s' % (member.name, self.name))
+
         self.photodb._cached_frozen_children = None
         self.photodb.cur.execute('INSERT INTO tag_group_rel VALUES(?, ?)', [self.id, member.id])
         if commit: