Zend Form Validator check record exists

User registration form is a good example for this validator. Checking the username exists or not.

Db_NoRecordExists validator helps to achieve this

$username = new Zend_Form_Element_Text('username',
 array('label' => 'Username',
     'required' => true,
     'filters' => array('StripTags', 'StringTrim'),         
     'validators' => array('NotEmpty', array('Db_NoRecordExists',false,array(
  'users_table','username_column', 'messages' => array( Zend_Validate_Db_Abstract::ERROR_RECORD_FOUND => '%value% already exists')
 )))));