Code review comment for lp:~sleepsonthefloor/nova/709057

> + self.assert_(len(group.instances) == 0)

Micro-nit: This is usually better written with self.assertEqual; not only is it more specific, it allows the unittest library to better pretty-print an assertion error since it has access to both the LHS and RHS of the expression.

Overall this patch lgtm, though this problem feels like it would be more elegantly solved using SQLAlchemy's cascading delete feature [1].

[1] http://www.sqlalchemy.org/docs/05/ormtutorial.html - search for cascade