Merge "Dont set _set_select_from() for alias object"

1 files changed, 5 insertions, 2 deletions

diff --git a/lib/sqlalchemy/orm/strategies.py b/lib/sqlalchemy/orm/strategies.py
index 4e3574b54..ffd47d17d 100644
--- a/lib/sqlalchemy/orm/strategies.py
+++ b/lib/sqlalchemy/orm/strategies.py
@@ -851,11 +851,14 @@ class SubqueryLoader(AbstractRelationshipLoader):
 
         # set a real "from" if not present, as this is more
         # accurate than just going off of the column expression
-        if not q._from_obj and orig_entity.mapper.isa(leftmost_mapper):
+        if not q._from_obj and orig_entity.is_mapper and \
+                orig_entity.mapper.isa(leftmost_mapper):
             q._set_select_from([orig_entity], False)
         target_cols = q._adapt_col_list(leftmost_attr)
 
-        # select from the identity columns of the outer
+        # select from the identity columns of the outer.  This will remove
+        # other columns from the query that might suggest the right entity
+        # which is why we try to _set_select_from above.
         q._set_entities(target_cols)
 
         distinct_target_key = leftmost_relationship.distinct_target_key