Add a constraint to skip fields during model and db sync tests

[Sudipta Biswas]

We would like to drop a few columns from the Models class and subsequently identify these fields for a database removal in the next openstack ('M') release.

However In nova/tests/unit/db/test_migrations.py - there's a test that does a check between the DB schema and models.py called "test_models_sync" - that matches the models and the db schema to ensure that there's no difference between the two.

We would like to have a 'whitelist' of attributes that are identified for cleanups from the models.py that could act as a constraint while doing this models_sync test. The attributes could be called "allow_missing_from_model" or "allow_missing_from_db" that would help us get past this error in the tests and at the same time help us maintain a list of attributes - that needs cleanup from the DB in the future releases.

The test failure could be observed on this review: https://review.openstack.org/#/c/192760/

[Sudipta Biswas (sbiswas7)]

Here's the error trace for the UT that fails:

2015-06-24 18:29:04.042 |
2015-06-24 18:29:04.042 | Captured traceback:
2015-06-24 18:29:04.042 | ~~~~~~~~~~~~~~~~~~~
2015-06-24 18:29:04.042 | Traceback (most recent call last):
2015-06-24 18:29:04.042 | File "/home/jenkins/workspace/gate-nova-python27/.tox/py27/local/lib/python2.7/site-packages/oslo_db/sqlalchemy/test_migrations.py", line 588, in test_models_sync
2015-06-24 18:29:04.042 | "Models and migration scripts aren't in sync:\n%s" % msg)
2015-06-24 18:29:04.042 | File "/home/jenkins/workspace/gate-nova-python27/.tox/py27/local/lib/python2.7/site-packages/unittest2/case.py", line 666, in fail
2015-06-24 18:29:04.042 | raise self.failureException(msg)
2015-06-24 18:29:04.042 | AssertionError: Models and migration scripts aren't in sync:
2015-06-24 18:29:04.042 | [ ( 'remove_column',
2015-06-24 18:29:04.043 | None,
2015-06-24 18:29:04.043 | 'instances',
2015-06-24 18:29:04.043 | Column('scheduled_at', DATETIME(), table=<instances>))]
2015-06-24 18:29:04.043 |

[Ann Taraday (akamyshnikova)]

This would be useful for Neutron implementation as well, as we have a numbers of errors that we have to skip always https://github.com/openstack/neutron/blob/master/neutron/tests/functional/db/test_migrations.py#L184-L205

[Ann Taraday (akamyshnikova)]

Does not this fixed by https://github.com/openstack/oslo.db/commit/94d583acde16860e7ca535bd7a960e8993cd12d2 ?