Forms Symfony为一组选项自定义表单验证程序

我有一个带有以下小部件和验证器的表单：

  $this->widgetSchema['num01'] = new sfWidgetFormChoice(array(
  'choices' => Doctrine_Core::getTable('ticket')->getTicketnum(),
  'expanded' => true, 'multiple' => true,
   ));

  $this->validatorSchema['num01'] = new sfValidatorChoice(array(
   'choices' =>
   array_keys(Doctrine_Core::getTable('ticket')->getTicketnum()), 'required' => true, 'multiple' => true,
   ));

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我遇到的问题是，它显示20个复选框，我想让他们只选择其中的5个。不多也不少。必须选中5个复选框。如何自定义代码以使其通过这种方式进行验证？谢谢

我试过这个，只是想看看它是否管用，它确实管用了。：）

$this->validatorSchema['num01'] = new sfValidatorChoice(array(
   'choices' =>
   array_keys(Doctrine_Core::getTable('ticket')->getTicketnum()), 'required' => true,
   'multiple' => true, 'max' => 5, 'min' => 5,
   ));