Entity form with pre-populated entity reference from URL

Question

I have two entities which are in a parent/child relationship. Now I want to add a operations link in the parent's list view so that I can show the child's entity form with the reference to the parent pre-populated.

Ideally, the URL would look something like /admin/myparent/123/mychild/add, where 123 is the ID of the parent entity to which the new child should have a reference. I have the link in place in the operations menu, but I can't figure out how to have my ChildEntityForm load in the parent entity based on the URL parameter. How do I do this in Drupal 8?

I found a couple of questions regarding this here on SO, but they were all about Drupal 7. In addition, the documentation (Using parameters in routes) regarding upcasting url parameters states this:

Note that this method of providing the entity data to a form is not used for entity forms, such as forms to add/edit/delete entity data.

So is there a solution to this, preferably one that would not require installing a third-party module?

Answer 1

The prepopulate module is now able to handle this in the usual way like

http://www.example.com/node/add/child?edit[field_parent_ref]=17

Answer 2

This is how i work for done that :

my ajax form call on my parent entity form :

 $form['bouton'] = array(

          '#type' => 'link',

          '#title' => $this->t('Ajouter un produit'),

          '#url' => Url::fromUri($base_url.'/admin/boutique/commandesLignes/add',array('query'=>array('idCommande'=>$entity->get('id')->value))),

          '#attributes' => [

              'class' => ['use-ajax'],

              'data-dialog-type' => 'modal',

              'data-dialog-options' => Json::encode([

                  'width' => 700,

              ]),

          ],

      );

Then in my commandesLignesAdd form (modal open with the link into parent entity), i get the parameter like this :

  public function buildForm(array $form, FormStateInterface $form_state) {
    /* @var $entity \Drupal\boutique\Entity\CommandesLignesEntity */
    $form = parent::buildForm($form, $form_state);
      $entity = $this->entity;

      $idCommande=\Drupal::request()->query->get('idCommande');

      $form_state->setTemporary(array('idCommandes'=>$idCommande));



    return $form;
  }

I set a temporary value into my $form_state and i use it into the submit call :

public function submitForm(array &$form, FormStateInterface $form_state)
{
    $commandes_id=(int)$form_state->getTemporaryValue('idCommandes');
    $form_state->setValue(array('commandes_id'),array('value'=>$commandes_id));

    parent::submitForm($form, $form_state);
}

My field reference name is commandes_id, and you need this field on your modal form, i use css for hidden it.