Rework AliasedClass __getattr__ to use top-level getattr()

Reworked the attribute mechanics used by :class:`.AliasedClass` to no
longer rely upon calling ``__getattribute__`` on the MRO of the wrapped
class, and to instead resolve the attribute normally on the wrapped class
using getattr(), and then unwrap/adapt that.  This allows a greater range
of attribute styles on the mapped class including special ``__getattr__()``
schemes; but it also makes the code simpler and more resilient in general.

Fixes: #4694
Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Change-Id: I28901e2472d3c21e881fe5cafa3b1d3af704fad8

1 files changed, 6 insertions, 2 deletions

diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index 321ab7d6f..74ebe40f6 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -346,10 +346,14 @@ def create_proxied_attribute(descriptor):
             )
 
         def __get__(self, instance, owner):
-            if instance is None:
+            retval = self.descriptor.__get__(instance, owner)
+            # detect if this is a plain Python @property, which just returns
+            # itself for class level access.  If so, then return us.
+            # Otherwise, return the object returned by the descriptor.
+            if retval is self.descriptor and instance is None:
                 return self
             else:
-                return self.descriptor.__get__(instance, owner)
+                return retval
 
         def __str__(self):
             return "%s.%s" % (self.class_.__name__, self.key)