Symfony DoctrineChoince Validator 'Where' condition

I want to make a "forgot password" form. For the username field I used the code:

    $this->form->setValidators(array(
        'username'   => new sfValidatorAnd(
            array(
                new sfValidatorString(array('min_length' => 4),
                    array(
                        'min_length' => 'Minim %min_length% caractere.',)),
                new sfValidatorDoctrineChoice(array(
                    'model'=>'Users',
                    'column'=>'username'), array(
                    'invalid' => 'Utilizatorul nu există.',
                    )),
            ),
            array(),
            arra开发者_开发技巧y(
                    'required' => 'Câmp obligatoriu',
            )
    ),

but for the email field I want to use the sfValidatorDoctrineChoice with a custom query, where the WHERE condition is: username = the 'username' field value gathered from the form.

What variable contains this value?

$this->form->getValue('username')

$request->getParameter('FormName') then the array contains all the submitted data.

$this->form->getValue('username') works only for bound and validated data.