How to access to an entity instance in a subform? - forms

Question

Summary
I want to access to an entity instance (of Media) inside a form related to an entity "Media" which is related to a form related to an entity "Project". How can I access to this entity instance ?
What i have tried
I already found on the net that some people talked about an event callback like this :
$builder->addEventListener(FormEvents::PRE_SET_DATA, function (FormEvent $event) use ($builder) {
/** #var YourEntity $data */
$data = $event->getData();
});
But that is only interesting when we had clicked on a 'add' button.
I want my entity on the first display of the page to show my image so that the user can see it and edit it if he wants to.
After, the user can add other images but it is another thing.
I also found some persons that say to put 'allow_add' to false, but it is not solving my problem.
The code
Here is my two forms.
The form Project :
class Project extends AbstractForm
{
use ArticleForm;
use StatusForm;
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add('medias', CollectionType::class, [
'entry_type' => Media::class,
'entry_options' => ['label' => false],
'allow_add' => true,
'allow_delete' => true
])
->add(
'teaser',
TextareaType::class,
[
'label' => 'label.teaser',
]
)
->add(
'public',
CheckboxType::class,
[
'label' => 'label.public',
]
);
$this->builderAddTitleAndBody($builder);
$this->builderAddStatus($builder, $options['data']);
$this->builderAddSave($builder);
}
public function configureOptions(OptionsResolver $resolver)
{
$resolver->setDefaults([
'data_class' => \App\Entity\Project::class,
]);
}
}
And the form Media :
class Media extends AbstractForm
{
use EntityForm;
public $formName = 'media';
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add(
'file',
FileType::class,
[
'data' => $this->entityFileInit($builder->getData(), 'file'),
'label' => 'label.picture',
'required' => false,
'mapped' => false,
'data_class' => null
]
)
->add(
'alt',
TextType::class,
[
'label' => 'label.alt',
'mapped' => false
]
);
}
public function configureOptions(OptionsResolver $resolver)
{
$resolver->setDefaults(
[
'data_class' => \App\Entity\Media::class,
]
);
}
}
Expected and actual results
I want Symfony to come at least to the template but Symfony tells me that $builder->getData() returns 'null'