HABTM form validation with CakePHP 2.x

I have a HABTM relation like : Post <-> Tag (a Post can have multiple Tag, and same the other way).

This work with the multiple checkbox selection generated by Cakephp. But I want to have at least one Tag for every Post and throw an error if someone try to insert an orphan.

I'm looking for the cleanest/most CakePHP alike way to do this.

This is more or less an update of this HABTM form validation in CakePHP question, as I get the same problem on my cakephp 2.7 (last cakephp 2.x for now with php 5.3 support at the date of 2016) and can't find a good way to do it.

Here are what I think is the best for now. It use the cakephp 3.x behaviour for HABTM validation.

I choose to only work in model, with the most generic code.

In your AppModel.php, set this beforeValidate() and afterValidate()

class AppModel extends Model {
   /** @var array set the behaviour to `Containable` */
 public $actsAs = array('Containable');

   /**
    * copy the HABTM post value in the data validation scope
    * from data[distantModel][distantModel] to data[model][distantModel]
    * @return bool true
    */
 public function beforeValidate($options = array()){
   foreach (array_keys($this->hasAndBelongsToMany) as $model){
     if(isset($this->data[$model][$model]))
       $this->data[$this->name][$model] = $this->data[$model][$model];
   }

   return true;
 }

   /**
    * delete the HABTM value of the data validation scope (undo beforeValidate())
    * and add the error returned by main model in the distant HABTM model scope
    * @return bool true
    */
 public function afterValidate($options = array()){
   foreach (array_keys($this->hasAndBelongsToMany) as $model){
     unset($this->data[$this->name][$model]);
     if(isset($this->validationErrors[$model]))
       $this->$model->validationErrors[$model] = $this->validationErrors[$model];
   }

   return true;
 }
}

After this, you can use your validation in you model like this :

class Post extends AppModel {

    public $validate = array(
        // [...]
        'Tag' => array(
              // here we ask for min 1 tag
            'rule' => array('multiple', array('min' => 1)),
            'required' => true,
            'message' => 'Please select at least one Tag for this Post.'
            )
        );

        /** @var array many Post belong to many Tag */
    public $hasAndBelongsToMany = array(
        'Tag' => array(
            // [...]
            )
        );
}

This answer use :