Add this model instance to a category.

        :param Category category: Category to add this object to
        :param User user: User heir who adds
        :return:

def add_to_category(self, category, user):
        """Add this model instance to a category.

        :param Category category: Category to add this object to
        :param User user: User heir who adds
        :return:
        """
        init_kwargs = {
            'category': category,
            'creator': user,
            'linked_object': self
        }
        tie = self.categories.model(**init_kwargs)  # That's a model of Tie.
        tie.save()
        return tie