When the instance is deleted, we propagate the deletion to the related
        collections, which will remove it from the related fields.

def delete(self):
        """
        When the instance is deleted, we propagate the deletion to the related
        collections, which will remove it from the related fields.
        """
        for related_collection_name in self.related_collections:
            related_collection = getattr(self, related_collection_name)
            related_collection.remove_instance()
        return super(RelatedModel, self).delete()