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

4 files changed, 89 insertions, 4 deletions

diff --git a/doc/build/changelog/unreleased_13/5000.rst b/doc/build/changelog/unreleased_13/5000.rst
new file mode 100644
index 000000000..2df2793b5
--- /dev/null
+++ b/doc/build/changelog/unreleased_13/5000.rst
@@ -0,0 +1,7 @@
+.. change::
+    :tags: bug, orm
+    :tickets: 5000
+
+    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.
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)
diff --git a/test/ext/test_associationproxy.py b/test/ext/test_associationproxy.py
index d515ccf65..e7cc6251b 100644
--- a/test/ext/test_associationproxy.py
+++ b/test/ext/test_associationproxy.py
@@ -18,6 +18,7 @@ from sqlalchemy.ext.declarative import declared_attr
 from sqlalchemy.orm import aliased
 from sqlalchemy.orm import clear_mappers
 from sqlalchemy.orm import collections
+from sqlalchemy.orm import composite
 from sqlalchemy.orm import configure_mappers
 from sqlalchemy.orm import create_session
 from sqlalchemy.orm import mapper
@@ -2330,6 +2331,74 @@ class DictOfTupleUpdateTest(fixtures.TestBase):
         )
 
 
+class CompositeAccessTest(fixtures.DeclarativeMappedTest):
+    @classmethod
+    def setup_classes(cls):
+        class Point(cls.Basic):
+            def __init__(self, x, y):
+                self.x = x
+                self.y = y
+
+            def __composite_values__(self):
+                return [self.x, self.y]
+
+            __hash__ = None
+
+            def __eq__(self, other):
+                return (
+                    isinstance(other, Point)
+                    and other.x == self.x
+                    and other.y == self.y
+                )
+
+            def __ne__(self, other):
+                return not isinstance(other, Point) or not self.__eq__(other)
+
+        class Graph(cls.DeclarativeBasic):
+            __tablename__ = "graph"
+            id = Column(
+                Integer, primary_key=True, test_needs_autoincrement=True
+            )
+            name = Column(String(30))
+
+            point_data = relationship("PointData")
+
+            points = association_proxy(
+                "point_data",
+                "point",
+                creator=lambda point: PointData(point=point),
+            )
+
+        class PointData(fixtures.ComparableEntity, cls.DeclarativeBasic):
+            __tablename__ = "point"
+
+            id = Column(
+                Integer, primary_key=True, test_needs_autoincrement=True
+            )
+            graph_id = Column(ForeignKey("graph.id"))
+
+            x1 = Column(Integer)
+            y1 = Column(Integer)
+
+            point = composite(Point, x1, y1)
+
+        return Point, Graph, PointData
+
+    def test_append(self):
+        Point, Graph, PointData = self.classes("Point", "Graph", "PointData")
+
+        g1 = Graph()
+        g1.points.append(Point(3, 5))
+        eq_(g1.point_data, [PointData(point=Point(3, 5))])
+
+    def test_access(self):
+        Point, Graph, PointData = self.classes("Point", "Graph", "PointData")
+        g1 = Graph()
+        g1.point_data.append(PointData(point=Point(3, 5)))
+        g1.point_data.append(PointData(point=Point(10, 7)))
+        eq_(g1.points, [Point(3, 5), Point(10, 7)])
+
+
 class AttributeAccessTest(fixtures.TestBase):
     def teardown(self):
         clear_mappers()
@@ -3080,7 +3149,7 @@ class MultiOwnerTest(
         is_(association_proxy_object.for_class(D, d2), inst2)
 
     def test_col_expressions_not_available(self):
-        D, = self.classes("D")
+        (D,) = self.classes("D")
 
         self._assert_raises_ambiguous(lambda: D.c_data == 5)
 
@@ -3269,7 +3338,7 @@ class ProxyHybridTest(fixtures.DeclarativeMappedTest, AssertsCompiledSQL):
         )
 
     def test_explicit_expr(self):
-        C, = self.classes("C")
+        (C,) = self.classes("C")
 
         s = Session()
         self.assert_compile(