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/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index b76a6f727..803d95feb 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -2826,10 +2826,10 @@ def configure_mappers():
             for mapper in list(_mapper_registry):
                 if getattr(mapper, '_configure_failed', False):
                     e = sa_exc.InvalidRequestError(
-                        "One or more mappers failed to initialize - "
+                        "One (%s) or more mappers failed to initialize - "
                         "can't proceed with initialization of other "
-                        "mappers.  Original exception was: %s"
-                        % mapper._configure_failed)
+                        "mappers. Original exception was: %s"
+                        % (mapper, mapper._configure_failed))
                     e._configure_failed = mapper._configure_failed
                     raise e
                 if not mapper.configured: