symfony/form validation constraint

gistfile1.php

// I am trying to put a validation constraint on an input field for inputting a username that it be unique - 
//if the name is already present in the database I want the validation to fail. 
//Is there an obvious way to go about this? Any hint in the right direction would be appreciated.


/*
Here's an example from one of my projects. This is from a registration 
form, to check if the email is already used. 

First, I make "app" a required option of my form type: 
*/
    public function setDefaultOptions(OptionsResolverInterface $resolver) 
    { 
        parent::setDefaultOptions($resolver); 
        $resolver->setRequired(array("app")); 
    } 

//Then while building the form, I can access it like: 

        $app = $options["app"]; 

//and when creating the email field, I attach a callback validator: 

        $builder->add("email", "email", array( 
            "label"         =>  "E-mail address", 
            "constraints"   =>  array( 
                new Assert\NotBlank(), 
                new Assert\Email(), 
                new Assert\Callback(array( 
                    "methods"   =>  array(function ($email, 
ExecutionContext $context) use ($app) { 
                        if ($app["db.user"]->findByEmail($email)) { 
                            $context->addViolation("Email taken"); 
                        } 
                    }), 
                )), 
            ), 
        )); 

//In my controller: 

    $form = $app["form.factory"]->create(new UserType(), null, 
array("app" => $app));