Adds a new user to the list of room members, if memeber doesn't exist already

async def add_member(self, db, user):
        ''' Adds a new user to the list of room members, if memeber doesn't exist already '''

        # use model's pk as query
        query = {self.primary_key: self.pk}
        # push review
        result = await db[self.get_collection_name()].update_one(
            filter=query,
            update={'$addToSet': {'members': DBRefField(User).to_python(user)}},
        )
        return result