fixes #7732 - specify join models as Rails 3.2.8 can't reset through associations

Rails 3.2.8 (used in our RPM installations) can't resolve models with through associations, so just specify the join models.  https://github.com/rails/rails/commit/3f0bc97912482c2da9e70683b029feb36de74c27 fixed the issue in 3.2.9.

Tested successfully on EL7.