Cakephp With MongoDB:无法在beforeSave函数中设置新变量

在将数据保存到MongoDB之前，即在

beforeSave（）方法中，如何将变量设置为集合

假设我有一个控制器

class BooksController extends AppController
{
    $model_name = 'Books';

    function save() {
        if($this->request->is('post')) {
        $data = $this->request->data['forms'];
        $this->$model_name->save($data);  // Already loaded the model
        }
    }
}

和模型

class Books extends AppModel 
{

    function beforeSave($options = array()) {
        $this->data['Books']['active'] = '1'; 
        // Tried this as well but found unstructured (key,value) pair `$this->data['active'] = '1';`
        //print_r($this->data) found correct format but active field is not getting saved in MongoDB
        return true;
    }

}

请让我知道任何进一步的澄清是必要的。
非常感谢您的帮助
 我是usign