DB: find_place_nth_parent()

mo/db.py

@@ -11,8 +11,9 @@ from sqlalchemy import \
     text, func, \
     create_engine, inspect, select
 from sqlalchemy.engine import Engine
-from sqlalchemy.orm import relationship, sessionmaker, Session, class_mapper, joinedload
+from sqlalchemy.orm import relationship, sessionmaker, Session, class_mapper, joinedload, aliased
 from sqlalchemy.orm.attributes import get_history
+from sqlalchemy.orm.query import Query
 from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.sql.expression import CTE
@@ -819,6 +820,15 @@ def get_place_descendants(place: Place, min_level: Optional[int] = None, max_lev
     return q.all()
 
 
+def filter_place_nth_parent(query: Query, place_attr: Any, n: int, parent_id: int) -> Query:
+    assert n >= 0
+    for _ in range(n):
+        pp = aliased(Place)
+        query = query.join(pp, pp.place_id == place_attr)
+        place_attr = pp.parent
+    return query.filter(place_attr == parent_id)
+
+
 def get_object_changes(obj):
     """ Given a model instance, returns dict of pending
     changes waiting for database flush/commit.