Add QueryableAttribute._impl_uses_objects accessor for AssociationProxy

Fixed regression where new association proxy system was still not proxying
hybrid attributes when they made use of the ``@hybrid_property.expression``
decorator to return an alternate SQL expression, or when the hybrid
returned an arbitrary :class:`.PropComparator`, at the expression level.
This involved futher generalization of the heuristics used to detect the
type of object being proxied at the level of :class:`.QueryableAttribute`,
to better detect if the descriptor ultimately serves mapped classes or
column expressions.

Fixes: #4690
Change-Id: I5b5300661291c94a23de53bcf92d747701720aa1

1 files changed, 2 insertions, 6 deletions

diff --git a/lib/sqlalchemy/ext/associationproxy.py b/lib/sqlalchemy/ext/associationproxy.py
index c725372c5..9a8294f3c 100644
--- a/lib/sqlalchemy/ext/associationproxy.py
+++ b/lib/sqlalchemy/ext/associationproxy.py
@@ -391,15 +391,11 @@ class AssociationProxyInstance(object):
             )
 
         attr = getattr(target_class, value_attr)
-        if (
-            not hasattr(attr, "_is_internal_proxy")
-            or attr._is_internal_proxy
-            and not hasattr(attr, "impl")
-        ):
+        if not hasattr(attr, "_is_internal_proxy"):
             return AmbiguousAssociationProxyInstance(
                 parent, owning_class, target_class, value_attr
             )
-        is_object = attr.impl.uses_objects
+        is_object = attr._impl_uses_objects
         if is_object:
             return ObjectAssociationProxyInstance(
                 parent, owning_class, target_class, value_attr