async def __call__(self, delta, old, new, model):
handler_name = 'on_{}_{}'.format(delta.entity, delta.type)
method = getattr(self, handler_name, self.on_change)
async def __call__(self, delta, old, new, model):
handler_name = 'on_{}_{}'.format(delta.entity, delta.type)
method = getattr(self, handler_name, self.on_change)