This method overrides django.db.models.deletion.Collector.collect.
parent instances. 'objs' must be a homogeneous iterable collection of model instances (e.g. a QuerySet). If 'collect_related' is True, related objects will be handled by their respective on_delete handler. If the call is the result of a cascade, 'source' should be the model that caused it and 'nullable' should be set to True, if the relation can be null. If 'reverse_dependency' is True, 'source' will be deleted before the current model, rather than after. (Needed for cascading to parent models, the one case in which the cascade follows the forwards direction of an FK rather than the reverse direction.) If 'keep_parents' is True, data of parent model's will be not deleted. If 'fail_on_restricted' is False, error won't be raised even if it's prohibited to delete such objects due to RESTRICT, that defers restricted object checking in recursive calls where the top-level call may need to collect more objects to determine whether restricted ones can be deleted.