When we have a model, save the relation in the database, to later create
        RelatedCollection objects in the related model

def _attach_to_model(self, model):
        """
        When we have a model, save the relation in the database, to later create
        RelatedCollection objects in the related model
        """
        super(RelatedFieldMixin, self)._attach_to_model(model)

        if model.abstract:
            # do not manage the relation if it's an abstract model
            return

        # now, check related_name and save the relation in the database

        # get related parameters to identify the relation
        self.related_name = self._get_related_name()
        self.related_to = self._get_related_model_name()

        # create entry for the model in the _relations list of the database
        if not hasattr(self.database, '_relations'):
            self.database._relations = {}
        self.database._relations.setdefault(self.related_to, [])

        # check unicity of related name for related model
        self._assert_relation_does_not_exists()

        # the relation didn't exists, we can save it
        relation = (self._model._name, self.name, self.related_name)
        self.database._relations[self.related_to].append(relation)