Don't assume m2o key is present in the dictionary

Fixed regression caused by new selectinload for many-to-one logic where
a primaryjoin condition not based on real foreign keys would cause
KeyError if a related object did not exist for a given key value on the
parent object.

Fixes: #4777
Change-Id: I4ba96318302be68abe0e8c3611684087a04a2322

1 files changed, 8 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/strategies.py b/lib/sqlalchemy/orm/strategies.py
index 82f40ea3b..8e8242c66 100644
--- a/lib/sqlalchemy/orm/strategies.py
+++ b/lib/sqlalchemy/orm/strategies.py
@@ -2380,7 +2380,12 @@ class SelectInLoader(AbstractRelationshipLoader, util.MemoizedSlots):
             }
 
             for key in chunk:
-                related_obj = data[key]
+                # for a real foreign key and no concurrent changes to the
+                # DB while running this method, "key" is always present in
+                # data.  However, for primaryjoins without real foreign keys
+                # a non-None primaryjoin condition may still refer to no
+                # related object.
+                related_obj = data.get(key, None)
                 for state, dict_, overwrite in our_states[key]:
                     if not overwrite and self.key in dict_:
                         continue
@@ -2430,6 +2435,8 @@ class SelectInLoader(AbstractRelationshipLoader, util.MemoizedSlots):
                         state, state_dict, collection[0]
                     )
                 else:
+                    # note that empty tuple set on uselist=False sets the
+                    # value to None
                     state.get_impl(self.key).set_committed_value(
                         state, state_dict, collection
                     )