Clarify mappers.configure_mappers error messagepr/298

Sometimes the message generated by configure_mappers lacks a reference
to the failing mapper, and only the mapper._configure_failed is shown. This
makes debugging problems extremely difficult. Example:

    sqlalchemy.exc.InvalidRequestError: One or more mappers failed to
    initialize - can't proceed with initialization of other mappers.  Original
    exception was: Class 'neutron.objects.router.Router' is not mapped

In the above failure, the actual object having a problem is FloatingIP, which
has a knock-on effect on the Router object when it fails to map. A more helpful
error message might look like this example:

    sqlalchemy.exc.InvalidRequestError: One (Mapper|FloatingIp|floatingip) or
    more mappers failed to initialize - can't proceed with initialization of
    other mappers.  Original exception was: Class
    'neutron.objects.router.Router' is not mapped

This patch adds the failing mapper name to the beginning of the message after
'One', as shown in the second example.

Change-Id: I9f23bfa90b26dde9229ab7ec812eec9ceae48153

1 files changed, 3 insertions, 3 deletions

diff --git a/test/ext/declarative/test_basic.py b/test/ext/declarative/test_basic.py
index ae1a85f8b..2fc9dcafd 100644
--- a/test/ext/declarative/test_basic.py
+++ b/test/ext/declarative/test_basic.py
@@ -772,9 +772,9 @@ class DeclarativeTest(DeclarativeTestBase):
         for i in range(3):
             assert_raises_message(
                 sa.exc.InvalidRequestError,
-                "^One or more mappers failed to initialize - "
-                "can't proceed with initialization of other "
-                "mappers.  Original exception was: When initializing.*",
+                "^One (Mapper|User|users) or more mappers failed to initialize"
+                " - can't proceed with initialization of other mappers. "
+                "Original exception was: When initializing.*",
                 configure_mappers)
 
     def test_custom_base(self):