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

1 files changed, 20 insertions, 0 deletions

diff --git a/test/orm/test_joins.py b/test/orm/test_joins.py
index 50fa86107..7f6e1b72e 100644
--- a/test/orm/test_joins.py
+++ b/test/orm/test_joins.py
@@ -6,6 +6,7 @@ from sqlalchemy import desc
 from sqlalchemy import exc as sa_exc
 from sqlalchemy import ForeignKey
 from sqlalchemy import func
+from sqlalchemy import inspect
 from sqlalchemy import Integer
 from sqlalchemy import lateral
 from sqlalchemy import literal_column
@@ -307,6 +308,25 @@ class JoinTest(QueryTest, AssertsCompiledSQL):
                 "WHERE users.name = :name_1",
             )
 
+    def test_join_relationship_propagate_attrs(self):
+        """test #6558"""
+
+        User = self.classes.User
+        users = self.tables.users
+
+        stmt = select(users).join(User.addresses)
+
+        eq_(
+            stmt._propagate_attrs,
+            {"compile_state_plugin": "orm", "plugin_subject": inspect(User)},
+        )
+
+        self.assert_compile(
+            stmt,
+            "SELECT users.id, users.name FROM users "
+            "JOIN addresses ON users.id = addresses.user_id",
+        )
+
     def test_invalid_kwarg_join(self):
         User = self.classes.User
         sess = fixture_session()