Ensure propagate_attrs available on PropComparator

Fixed regression caused by just-released performance fix mentioned in #6550
where a query.join() to a relationship could produce an AttributeError if
the query were made against non-ORM structures only, a fairly unusual
calling pattern.

In this fix, since we are no longer going through the production
of ``__clause_element__()`` for Cls.some_relationship, I assumed we
just throw this object away completely but I missed the one little
bit where we might be getting ``_propagate_attrs`` from it.
So we implement ``_propagate_attrs`` on ``PropComparator`` as well,
since this is easy to define.

Fixes: #6558
Change-Id: If781bf844e7e3d3b0841aff1c3668e9d6af9f097

2 files changed, 15 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/interfaces.py b/lib/sqlalchemy/orm/interfaces.py
index ed38becf7..c9a601f99 100644
--- a/lib/sqlalchemy/orm/interfaces.py
+++ b/lib/sqlalchemy/orm/interfaces.py
@@ -426,6 +426,18 @@ class PropComparator(operators.ColumnOperators):
         return inspect(self._parententity).mapper
 
     @property
+    def _propagate_attrs(self):
+        # this suits the case in coercions where we don't actually
+        # call ``__clause_element__()`` but still need to get
+        # resolved._propagate_attrs.  See #6558.
+        return util.immutabledict(
+            {
+                "compile_state_plugin": "orm",
+                "plugin_subject": self._parentmapper,
+            }
+        )
+
+    @property
     def adapter(self):
         """Produce a callable that adapts column expressions
         to suit an aliased version of this comparator.
diff --git a/lib/sqlalchemy/sql/coercions.py b/lib/sqlalchemy/sql/coercions.py
index 82068d768..ea2289a9d 100644
--- a/lib/sqlalchemy/sql/coercions.py
+++ b/lib/sqlalchemy/sql/coercions.py
@@ -884,6 +884,9 @@ class JoinTargetImpl(RoleImpl):
         self, original_element, resolved, argname=None, legacy=False, **kw
     ):
         if isinstance(original_element, roles.JoinTargetRole):
+            # note that this codepath no longer occurs as of
+            # #6550, unless JoinTargetImpl._skip_clauseelement_for_target_match
+            # were set to False.
             return original_element
         elif legacy and isinstance(resolved, str):
             util.warn_deprecated_20(