How to return json message inside controller?

I have a controller which takes in an id and checks whether this id exists inside the Model. If it does not exist, I would like the controller to return a validation error message "id not found" in json when it returns the HTTP response. This controller takes in a normal HTTP POST which is not in json.

How can this be done in Cakephp 2.4.5?

My controller code looks something like this;

public function controller_function($id=null)  {      if ($this->request->is('post'))       {         $field=$this->request->data['Model']['field'];         $Model_id = $this->Model->findFieldID($field);         if (empty($Model_id) ) //record not found. Return validation error         {             //Send validation error back in JSON. How??         }      }  }

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