Symfony2 - Validation not working for embedded Form Type

I spent an age searching and found that it was adding 'cascade_validation' => true to the setDefaults() array in my parent type's class that fixed it (as mentioned already in the thread). This causes the entity constraint validation to trigger in the child types shown in the form. e.g.

public function setDefaultOptions(OptionsResolverInterface $resolver)
{
    $resolver->setDefaults(array(            
        ...
        'cascade_validation' => true,
    ));
}

For collections, also make sure to add 'cascade_validation' => true to the $options array for the collection field on the form. e.g.

$builder->add('children', 'collection', array(
    'type'         => new ChildType(),
    'cascade_validation' => true,
));

This will have the UniqueEntity validation take place as it should in the child entity used in the collection.

A note to those using Symfony 3.0 and up: the cascade_validation option has been removed. Instead, use the following for embedded forms:

$builder->add('embedded_data', CustomFormType::class, array(
    'constraints' => array(new Valid()),
));

Sorry for adding to this old thread with a slightly off-topic answer (Symfony 3 vs. 2), but finding this information here would have saved me a few hours today.

According to form type documentation you can also use Valid constraint instead of cascade_validation option.

$builder->add('children', 'collection', array(
    'type'        => new ChildType(),
    'constraints' => array(new Valid()),
));

Example from the owner entity:

/**
 * @var Collection
 *
 * @ORM\OneToMany(targetEntity="Child", ...)
 * @Assert\Valid()
 */
private $children