Fixes for uselist=True with m2o relationships

Fixed bug where a many-to-one relationship that specified ``uselist=True``
would fail to update correctly during a primary key change where a related
column needs to change.

Fixed bug where the detection for many-to-one or one-to-one use with a
"dynamic" relationship, which is an invalid configuration, would fail to
raise if the relationship were configured with ``uselist=True``.  The
current fix is that it warns, instead of raises, as this would otherwise be
backwards incompatible, however in a future release it will be a raise.

Fixes: #4772
Change-Id: Ibd5d2f7329ff245c88118e2533fc8ef42a09fef3

1 files changed, 12 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/dynamic.py b/lib/sqlalchemy/orm/dynamic.py
index 20544dbbe..3218b9fa3 100644
--- a/lib/sqlalchemy/orm/dynamic.py
+++ b/lib/sqlalchemy/orm/dynamic.py
@@ -14,6 +14,7 @@ basic add/delete mutation.
 
 from . import attributes
 from . import exc as orm_exc
+from . import interfaces
 from . import object_mapper
 from . import object_session
 from . import properties
@@ -36,6 +37,17 @@ class DynaLoader(strategies.AbstractRelationshipLoader):
                 "many-to-one/one-to-one relationships and/or "
                 "uselist=False." % self.parent_property
             )
+        elif self.parent_property.direction not in (
+            interfaces.ONETOMANY,
+            interfaces.MANYTOMANY,
+        ):
+            util.warn(
+                "On relationship %s, 'dynamic' loaders cannot be used with "
+                "many-to-one/one-to-one relationships and/or "
+                "uselist=False.  This warning will be an exception in a "
+                "future release." % self.parent_property
+            )
+
         strategies._register_attribute(
             self.parent_property,
             mapper,