Generalize DescriptorProps.uses_objects

Previously, uses_objects was specific to the SynonymAttribute;
generalize it so that it defaults to False for other DescriptorProps.
Immediate fix is against CompositeProperty.

Fixed regression introduced in 1.3.0 related to the association proxy
refactor in :ticket:`4351` that prevented :func:`.composite` attributes
from working in terms of an association proxy that references them.

Add test coverage for association proxies that refer to Composite
attributes as endpoints.

Fixes: #5000
Change-Id: Iea6fb1bd3314d861a9bc22491b0ae1e6c5e6340d

2 files changed, 11 insertions, 2 deletions

diff --git a/lib/sqlalchemy/orm/descriptor_props.py b/lib/sqlalchemy/orm/descriptor_props.py
index 3be5502ce..f22dd15c7 100644
--- a/lib/sqlalchemy/orm/descriptor_props.py
+++ b/lib/sqlalchemy/orm/descriptor_props.py
@@ -31,6 +31,8 @@ class DescriptorProperty(MapperProperty):
 
     doc = None
 
+    uses_objects = False
+
     def instrument_class(self, mapper):
         prop = self
 
@@ -41,7 +43,7 @@ class DescriptorProperty(MapperProperty):
 
             @property
             def uses_objects(self):
-                return getattr(mapper.class_, prop.name).impl.uses_objects
+                return prop.uses_objects
 
             def __init__(self, key):
                 self.key = key
@@ -637,6 +639,10 @@ class SynonymProperty(DescriptorProperty):
 
         util.set_creation_order(self)
 
+    @property
+    def uses_objects(self):
+        return getattr(self.parent.class_, self.name).impl.uses_objects
+
     # TODO: when initialized, check _proxied_property,
     # emit a warning if its not a column-based property
 
diff --git a/lib/sqlalchemy/testing/fixtures.py b/lib/sqlalchemy/testing/fixtures.py
index e2237fb17..04d09f8dd 100644
--- a/lib/sqlalchemy/testing/fixtures.py
+++ b/lib/sqlalchemy/testing/fixtures.py
@@ -388,7 +388,10 @@ class DeclarativeMappedTest(MappedTest):
             cls=DeclarativeBasic,
         )
         cls.DeclarativeBasic = _DeclBase
-        fn()
+
+        # sets up cls.Basic which is helpful for things like composite
+        # classes
+        super(DeclarativeMappedTest, cls)._with_register_classes(fn)
 
         if cls.metadata.tables and cls.run_create_tables:
             cls.metadata.create_all(config.db)