'Restricts the query requiring matches in the given model.
This builds upon previous additions, so you can limit to multiple models
by chaining this method several times.'

def add_model(self, model):

    if (not isinstance(model, ModelBase)):
        raise AttributeError(u'The    model    being    added    to    the    query    must    derive    from    Model.')
    self.models.add(model)
