Validation is not working with many_many Multi Class Adding

[bhofstaetter]

I'm using multi class adding on a page to add data objects to a many_many relation.
The data object's getCMSValidator() method only gets called after the data object is added to the relation. So the validation only takes place after the object is written and if you hit the save button again.

[bhofstaetter]

for everyone else with this issue. a workaround would be to use the validate() methode like that

  public function validate() {
    $result = parent::validate();

    if(!$this->ID) {
      $manyRelations = array_merge($this->many_many(), $this->has_many());
      $oneRelations = $this->has_one();
      $fields = $this->getCMSValidator()->getRequired();

      foreach($fields as $field) {
        if(array_key_exists($field, $manyRelations)) {
          if(!$this->$field()->first()) {
            $result->error('"' . $field . '" muss ausgefüllt werden');
          }
        } else if (array_key_exists($field, $oneRelations)) {
          if(!$this->$field()->exists()) {
            $result->error('"' . $field . '" muss ausgefüllt werden');
          }
        } else {
          if(!$this->$field) {
            $result->error('"' . $field . '" muss ausgefüllt werden');
          }
        }
      }
    }

    return $result;
  }

Updated my workaround to cover has_many, many_many and has_one realtions

[user121216]

In my case your workaround is not working. The $manyRelations are always empty. I'm using silverstripe 3.2.
getCMSValidator is partially working because it detects the missing relations. But I still got a strange behaviour. Validations in validate() will be only executed if getCMSValidator is successfully.
E.g. the user got an error for missing relation (getCMSValidator) and after fixing this, the user got further error messages (validate()).

Can somebody describe a correct implementation to use validate() and a if condition for non empty relation objects?