Symfony2 embedded forms are nullified despite of cascade_validation being false - forms

Question

In a form (ContractType) I need the association to a Customer, which itself has an association to a Company and a BillingAddress. But the data itself of these 3 "nested entities" should not be editable at all in the contract view. That's why the field for the customer ManyToOne-Relation in the contract is hidden.
However for display purposes I need the whole customer-object in the form too (incl. the underlying company and billing address) - but as read-only. This is why I added an additional field customerFullObject to the contract form-type - which is not mapped (mapped => false). As it is an unmapped field, I set the data via a form event.
Now, when I edit the contract data, the company- and billing-address-data of the customer is nullified.
If I remove/comment the whole customerFullObject part, everything is fine.
I don't understand, how/why an unmapped field can change the data of an entity.
How can I prevent Symfony from this? Or is there a better "pattern"/way to achieve what I want? I couldn't find anything in the docs.
Below some code snippets from the model (removed namespaces for briefness):
Contract.php
/**
* #ORM\Entity
*/
class Contract {
/**
* #var Customer
* #ORM\ManyToOne(targetEntity="Customer", inversedBy="contracts")
* #Assert\Type(type="Customer")
* #Assert\NotBlank(message = "customer.notBlank")
**/
private $customer;
// Other fields left out as irrelevant.
}
Customer.php
/**
* #ORM\Entity
*/
class Customer {
/**
* #var Company
* #ORM\OneToOne(targetEntity="Company")
* #Assert\NotBlank(message="company.notBlank")
**/
private $company;
/**
* #var Address
* #ORM\OneToOne(targetEntity="Address")
* #Assert\NotBlank(message="billingAddress.notBlank")
**/
private $billingAddress;
/**
* #var ArrayCollection
* #ORM\OneToMany(targetEntity="Contract", mappedBy="customer", orphanRemoval=true, cascade={"all"})
**/
private $contracts;
// Other fields left out as irrelevant.
}
ContractType.php
class ContractType extends AbstractType {
public function buildForm(FormBuilderInterface $builder, array $options) {
$builder->add(
'customer',
'customerHiddenType',
array(
'label' => 'contract.customer',
)
);
$builder->add(
'customerFullObject',
'customerType',
array(
'cascade_validation' => false,
'mapped' => false
)
);
$builder->addEventListener(FormEvents::POST_SET_DATA, function (FormEvent $event) {
$form = $event->getForm();
$editMode = $form->getConfig()->getOption('editMode');
if (!empty($editMode) && $editMode === RequestAction::CREATE()) {
$form->remove('customerFullObject');
} else {
$form->get('customerFullObject')->setData($event->getData()->getCustomer());
}
}
);
// Other children left out as irrelevant.
}
public function setDefaultOptions(OptionsResolverInterface $resolver) {
$resolver->setDefaults(
array(
'data_class' => 'Contract',
'cascade_validation' => true
)
);
}
public function getName() {
return 'contractType';
}
}
CustomerType.php
class CustomerType extends AbstractType {
public function buildForm(FormBuilderInterface $builder, array $options) {
$builder->add(
'company',
'companyType',
array(
'cascade_validation' => false,
'label' => 'customer.company'
)
);
$builder->add(
'billingAddress',
'addressType',
array(
'cascade_validation' => false,
'label' => 'customer.billingAddress'
)
);
// Other children left out as irrelevant.
}
public function setDefaultOptions(OptionsResolverInterface $resolver) {
$resolver->setDefaults(
array(
'data_class' => 'Customer',
'cascade_validation' => true
)
);
}
public function getName() {
return 'customerType';
}
}