Django 1.7+ changed some variables around for relation fields, turning

parent_model into model and model into related_model. Especially

in the case of model, the meaning has changed significantly. We now

have some compatibility functions that check for the right thing when

attempting to access the proper variable. This restores test

functionality, bringing compatibility to Django 1.6 and 1.7/1.8.

These unit tests pass on Django 1.6 and 1.8.