/**
    * Add custom validation to the form
    *
    * @access public
    * @return RequiredFields
    */
   public function getCMSValidator() {
      return new RequiredFields('Topic', 'StartDate', 'EndDate');
   }

class Project extends DataObject {

   // fields, methods, etc

   public function getCMSValidator() {
      return new Project_Validator();
   }

}

class Project_Validator extends RequiredFields {

   protected $customRequired = array('Number');

   /**
    * Constructor
    */
   public function __construct() {
      $required = func_get_args();
      if(isset($required[0]) && is_array($required[0])) {
         $required = $required[0];
      }
      $required = array_merge($required, $this->customRequired);

      parent::__construct($required);
   }
   
   function php($data) {
      $valid = parent::php($data);
      $number_SQL = Convert::raw2sql($data['Number']);
      if(isset($_REQUEST['ctf']['childID'])) {
         $id = $_REQUEST['ctf']['childID'];
      } elseif(isset($_REQUEST['ID'])) {
         $id = $_REQUEST['ID'];
      } else {
         $id = null;
      }
      
      $project = DataObject::get("Project", "`Number` = '{$number_SQL}'");
      
      if($id) {
         // Existing project, check that number hasn't changed.
         $project = DataObject::get_by_id("Project", $id);
         if($project->Number != $data['Number']) {
            $this->validationError("Number", "Sorry, you cannot change the project number.");
            $valid = false;
         }
      } else {
         // New project, check it doesn't already exist.
         if(DataObject::get("Project", "`Number` = '{$number_SQL}'")) {
            $this->validationError("Number", "Sorry, this project number already exists.");
            $valid = false;
         }
      }
      
      return $valid;
   }
}