Added SubqueryLoader to load model from query alias

gino/loader.py

@@ -160,6 +160,13 @@ def none_as_none(self, enabled=True):
         return self
 
 
+class SubqueryLoader(ModelLoader):
+    def __init__(self, model, subquery, **extras):
+        super().__init__(model, **extras)
+        self.columns = [subquery.corresponding_column(column) for column
+                        in model]
+
+
 class AliasLoader(ModelLoader):
     def __init__(self, alias, *columns, **extras):
         super().__init__(alias, *columns, **extras)

tests/test_loader.py

@@ -151,6 +151,27 @@ def loader(row, context):
         assert u.team.parent.name == user.team.parent.name
 
 
+async def test_adjanency_list_on_nested_load(user):
+    subquery = db.select(User).alias()
+    base_query = subquery.outerjoin(Team).select()
+
+    query = base_query.execution_options(loader=(User.load('id')))
+    u = await query.gino.first()
+    # Because here arrives team_id, not user_id, and replaces it
+    assert u.id is None
+
+    from gino.loader import SubqueryLoader
+    query = base_query.execution_options(loader=SubqueryLoader(User, subquery, team=Team))
+    u = await query.gino.first()
+    assert u.id == user.id
+    assert u.realname == user.realname
+    assert u.nickname == user.nickname
+
+    assert isinstance(u.team, Team)
+    assert u.team.id == user.team.id
+    assert u.team.name == user.team.name
+
+
 async def test_adjacency_list_query_builder(user):
     group = Team.alias()
     u = await User.load(team=Team.load(parent=group.on(