warn or deprecate for auto-aliasing in joins

An extra layer of warning messages has been added to the functionality
of :meth:`_orm.Query.join` and the ORM version of
:meth:`_sql.Select.join`, where a few places where "automatic aliasing"
continues to occur will now be called out as a pattern to avoid, mostly
specific to the area of joined table inheritance where classes that share
common base tables are being joined together without using explicit aliases.
One case emits a legacy warning for a pattern that's not recommended,
the other case is fully deprecated.

The automatic aliasing within ORM join() which occurs for overlapping
mapped tables does not work consistently with all APIs such as
``contains_eager()``, and rather than continue to try to make these use
cases work everywhere, replacing with a more user-explicit pattern
is clearer, less prone to bugs and simplifies SQLAlchemy's internals
further.

The warnings include links to the errors.rst page where each pattern is
demonstrated along with the recommended pattern to fix.

* Improved the exception message generated when configuring a mapping with
  joined table inheritance where the two tables either have no foreign key
  relationships set up, or where they have multiple foreign key relationships
  set up. The message is now ORM specific and includes context that the
  :paramref:`_orm.Mapper.inherit_condition` parameter may be needed
  particularly for the ambiguous foreign keys case.

* Add explicit support in the _expect_warnings() assertion for nested
  _expect_warnings calls

* generalize the NoCache fixture, which we also need to catch warnings
  during compilation consistently

* generalize the __str__() method for the HasCode mixin so all warnings
  and errors include the code link in their string

Fixes: #6974
Change-Id: I84ed79ba2112c39eaab7973b6d6f46de7fa80842

2 files changed, 2 insertions, 15 deletions

diff --git a/test/aaa_profiling/test_memusage.py b/test/aaa_profiling/test_memusage.py
index 6462ab9f1..1c08c30a2 100644
--- a/test/aaa_profiling/test_memusage.py
+++ b/test/aaa_profiling/test_memusage.py
@@ -1141,7 +1141,7 @@ class MemUsageWBackendTest(EnsureZeroed):
 
         @profile_memory()
         def go():
-            s = table2.select().subquery()
+            s = aliased(Bar, table2.select().subquery())
             sess = session()
             sess.query(Foo).join(s, Foo.bars).all()
             sess.rollback()
diff --git a/test/aaa_profiling/test_orm.py b/test/aaa_profiling/test_orm.py
index 7de1811ae..727d48fca 100644
--- a/test/aaa_profiling/test_orm.py
+++ b/test/aaa_profiling/test_orm.py
@@ -18,27 +18,14 @@ from sqlalchemy.orm import relationship
 from sqlalchemy.orm import selectinload
 from sqlalchemy.orm import Session
 from sqlalchemy.orm import sessionmaker
-from sqlalchemy.testing import config
 from sqlalchemy.testing import fixtures
 from sqlalchemy.testing import profiling
 from sqlalchemy.testing.fixtures import fixture_session
+from sqlalchemy.testing.fixtures import NoCache
 from sqlalchemy.testing.schema import Column
 from sqlalchemy.testing.schema import Table
 
 
-class NoCache(object):
-    run_setup_bind = "each"
-
-    @classmethod
-    def setup_test_class(cls):
-        cls._cache = config.db._compiled_cache
-        config.db._compiled_cache = None
-
-    @classmethod
-    def teardown_test_class(cls):
-        config.db._compiled_cache = cls._cache
-
-
 class MergeTest(NoCache, fixtures.MappedTest):
     __requires__ = ("python_profiling_backend",)