Validate Symfony Form Type with Entity method

Entity.php

<?php
....
    /**
     * Validate Method
     *
     * @param ExecutionContext $context Execution context
     *
     * @return void
     */
    public function validateMethod(ExecutionContext $context)
    {
        $context->addViolationAt('fields', 'Error Message.');
    }
....

FormType.php

<?php
....
    /**
     * {@inheritdoc}
     */
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            ....
            'constraints' => array(
                new Assert\Callback(array('methods' => array('validateMethod'))),
            ),
        ));
    }
....