<?php

// main entity
class Author {
    /**
     * @Assert\NotBlank(groups={"Default", "bla"})
     */
    protected $name;

    /**
     * @var Address
     */
    protected $address;

    // ...
}

// child entity
class Address {
    /**
     * @Assert\NotBlank(groups={"Default", "bla"})
     */
    protected $street;

    // ...
}

// main form
class CreateAuthorFormType {
    public function buildForm(FormBuilder $builder, array $options) {
        $builder->add('name');
        $builder->add('address', 'form_type_address', array(
            'data_class' => get_class(new Address()),
        ));
    }
}

// child form
class EditAddressFormType {
    public function buildForm(FormBuilder $builder, array $options) {
        $builder->add('street');
    }
    public function getName() {
        return 'form_type_address';
    }
}

// in a Controller
$author = new Author();
$author->setAddress(new Address());
$createAuthorForm = $this->createForm(new CreateAuthorFormType(), $author, array(
    'validation_groups' => 'bla',
    'cascade_validation' => true,
));
// ...
if ($createAuthorForm->isValid()) {
    // ISSUE: will be valid even if street is empty
}