public function save($model, $id, $data)
    {
        if (is_null($id)) { // insert
            $model->data = $this->sendCollectionRequest($model, $this->insert_mode, $data);
            $model->id = $model->data ? $model->data[$model->id_field] : null;
        } else { // update
            $model->data = $this->sendItemRequest($model, $id, $this->update_mode, $data);
            $model->id = $model->data ? $model->data[$model->id_field] : null;
        }

        return $model->id;
    }