Look up adapter info for previous left side in chained query.join()

Fixed issue where chaining multiple join elements inside of
:meth:`.Query.join` might not correctly adapt to the previous left-hand
side, when chaining joined inheritance classes that share the same base
class.

Change-Id: I4b846430b7362912dbebf50599ec15a1eb978fd4
Fixes: #3505

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/query.py b/lib/sqlalchemy/orm/query.py
index 74d3079af..98747c680 100644
--- a/lib/sqlalchemy/orm/query.py
+++ b/lib/sqlalchemy/orm/query.py
@@ -2244,6 +2244,13 @@ class Query(object):
 
                 left_entity = onclause._parententity
 
+                alias = self._polymorphic_adapters.get(left_entity, None)
+                # could be None or could be ColumnAdapter also
+                if isinstance(alias, ORMAdapter) and \
+                        alias.mapper.isa(left_entity):
+                    left_entity = alias.aliased_class
+                    onclause = getattr(left_entity, onclause.key)
+
                 prop = onclause.property
                 if not isinstance(onclause, attributes.QueryableAttribute):
                     onclause = prop