MyModel.php

<?php

namespace Acme\DemoBundle\Model;

use Symfony\Component\Validator\ExecutionContextInterface;

class MyModel
{
    public $myProperty;
    
    public function validateProperty(ExecutionContextInterface $context)
    {
        $context->addViolationAt('myProperty', 'Error');
    }
}

MyModelType.php

<?php

namespace Acme\Bundle\DemoBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
use Symfony\Component\Validator\Constraints\Callback;

class MyModelType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('myProperty', 'text');
    }
    
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'Acme\DemoBundle\Model\MyModel',
            'constraints' => array(
                new Callback(array(
                    'callback' => 'validateProperty',
                ),
            ),
        ));
    }
    
    public function getName()
    {
        return 'my_model_form';
    }
}