Symfony CollectionType: Merge new entries - forms

Question

My symfony form represents an entity Mail which has a one to many relation with another entity called Attachment. Therefore, the MailType form contains a CollectionType field for embedding its AttachmentType forms:
$builder
->add('attachments', CollectionType::class, [
'entry_type' => AttachmentType::class,
'allow_add' => true,
'allow_delete' => false,
'by_reference' => false,
]);
My view will only send new attachments to my Symfony backend. So when storing the form data into the database, I only want to add new attachments of the mail and do not touch any existing attachments.
Unfortunately, Symfony / Doctrine behave differently: If n attachments are contained in the form data then n first existing attachments are overwritten by those new attachments:
existing attachments (in DB): [old1, old2, old3]
new attachments (contained by HTTP request): [new1, new2]
desired result in DB: [old1, old2, old3, new1, new2]
actual result in DB: [new1, new2, old3]
How can I achieve this? I thought by_reference => false will cause the addAttachment method to be called, so I also expected this to work out-of-the-box.
My Mail entity code:
class Mail {
/**
* #ORM\OneToMany(targetEntity="AppBundle\Entity\Attachment", mappedBy="mail", cascade={"persist", "remove"})
*/
protected $attachments;
...
public function addAttachment(\AppBundle\Entity\ttachment $attachment) {
$attachment->setMail($this);
$this->attachments[] = $attachment;
return $this;
}
}
My controller code processing the form:
// $mail = find mail in database
$form = $this->createForm(MailType::class, $mail);
$form->handleRequest($request);
if ($form->isValid()) {
$mail = $form->getData();
$em = $this->getDoctrine()->getManager();
$em->persist($mail);
$em->flush();
}

Answer 1

There are a couple of ways to do what you want. The simplest would be to give empty data or new attachment entities to your form field:
$builder
->add('attachments', CollectionType::class, [
'entry_type' => AttachmentType::class,
'allow_add' => true,
'allow_delete' => true,
'by_reference' => false,
'data' => [new Attachment()] // or add more or []
]);
Then in your Mail entity:
public function addAttachment(Attachment $attachment) {
$attachment->setMail($this);
$this->attachments[] = $attachment;
return $this;
}
public function removeAttachment(Attachment $attachment) {
return $this;
}
If you are using removeAttachment for some other functionality and you want to actually remove the Attachment, you can take advantage of the property_path settings of the form field:
'property_path' => 'appendAttachments'
and create addAppendAttachment and removeAppendAttachment:
public function addAppendAttachment(Attachment $attachment) {
$attachment->setMail($this);
$this->attachments[] = $attachment;
return $this;
}
public function removeAppendAttachment(Attachment $attachment) {
return $this;
}