Add Query.lazy_load_from attribute for sharding

Added new attribute :attr:`.Query.lazy_loaded_from` which is populated
with an :class:`.InstanceState` that is using this :class:`.Query` in
order to lazy load a relationship.  The rationale for this is that
it serves as a hint for the horizontal sharding feature to use, such that
the identity token of the state can be used as the default identity token
to use for the query within id_chooser().

Also repaired an issue in the :meth:`.Result.with_post_criteria`
method added in I899808734458e25a023142c2c5bb37cbed869479
for :ticket:`4128` where the "unbake subquery loaders" version was calling
the post crtieria functions given the :class:`.Result` as the argument
rather than applying them to the :class:`.Query`.

Change-Id: I3c0919ce7fd151b80fe2f9b5f99f60df31c2d73d
Fixes: #4243

1 files changed, 15 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/assertions.py b/lib/sqlalchemy/testing/assertions.py
index 69d43c92f..e42376921 100644
--- a/lib/sqlalchemy/testing/assertions.py
+++ b/lib/sqlalchemy/testing/assertions.py
@@ -518,6 +518,21 @@ class AssertsExecutionResults(object):
         self.assert_sql_execution(
             db, callable_, assertsql.CountStatements(count))
 
+    def assert_multiple_sql_count(self, dbs, callable_, counts):
+        recs = [
+            (self.sql_execution_asserter(db), db, count)
+            for (db, count) in zip(dbs, counts)
+        ]
+        asserters = []
+        for ctx, db, count in recs:
+            asserters.append(ctx.__enter__())
+        try:
+            return callable_()
+        finally:
+            for asserter, (ctx, db, count) in zip(asserters, recs):
+                ctx.__exit__(None, None, None)
+                asserter.assert_(assertsql.CountStatements(count))
+
     @contextlib.contextmanager
     def assert_execution(self, db, *rules):
         with self.sql_execution_asserter(db) as asserter: