'Save a new document.
Helper method, should only be used inside save().'

def _save_create(self, doc, force_insert, write_concern):

    collection = self._get_collection()
    if force_insert:
        return collection.insert(doc, **write_concern)
    object_id = collection.save(doc, **write_concern)
    if ((not object_id) and (pymongo.version_tuple == (3, 0))):
        pk_as_mongo_obj = self._fields.get(self._meta['id_field']).to_mongo(self.pk)
        object_id = (self._qs.filter(pk=pk_as_mongo_obj).first() and self._qs.filter(pk=pk_as_mongo_obj).first().pk)
    return object_id
