Symfony2, call part of a form in an Ajax query - symfony

Question

I have a form in Symfony.
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add('volume', IntegerType::class, array())
;
$builder->add('product', ChoiceType::class, array(
'data' => '1',
'mapped' => false,
'choices' => array(
'1' => 'One',
'2' => 'Two',
),
)
);
$builder->add('deliveryType', ChoiceType::class, array(
'expanded' => true,
'data' => 1,
'choices' => array(
Command::STANDARD_DELIVERY => 'Standard',
Command::EXPRESS_DELIVERY => 'Express',
),
));
}
But the field deliveryType is loaded in another template called with an ajax request.
But of corse I have this error:
Variable "form" does not exist in ... at line 26
How can I put a part of a form in another template?
Here is my main template:
<div class="panel-body">
{{ form_start(form)}}
{{ form_errors(form) }}
<div class="row">
<div class="col-sm-12">
{{ form_row(form.product) }}
{{ form_row(form.volume, {'attr': {'class': 'form-control'}}) }}
{{ form_row(form.town) }}
</div>
</div>
<div id="content-delivery-range"><!-- Block loaded by ajax --></div>
<br>
<button type="submit" class="btn btn-primary btn-block">Continuer </button>
{{ form_end(form) }}
</div>
And now my template called with an Ajax request:
{{ deliveryRange|first|date('d M') }}
{{ deliveryRange|last|date('d M') }}
{% for d in deliveryRange %}
{{ d|date('l') }} <br>
<small>{{ d|intl_date('d') }} {{ d|date('M') }}</small>
{% endfor %}
{{ form_row(form.deliveryType) }}

Answer 1

It might be easier to include the "deliveryType" form element as part of the initial form render.
Then, your ajax call could just return the necessary values which you insert using jquery/javascript/etc.

Answer 2

Found it:
I have used an event listener with PRE_SUBMIT:
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add('volume', IntegerType::class, array())
;
$builder->add('product', ChoiceType::class, array(
'data' => '1',
'mapped' => false,
'choices' => array(
'1' => 'One',
'2' => 'Two',
),
)
);
$builder->addEventListener(FormEvents::PRE_SUBMIT, array($this, 'onPreSubmit'));
}
public function onPreSubmit(FormEvent $event)
{
$form = $event->getForm();
$form->add('deliveryType', ChoiceType::class, array(
'expanded' => true,
'data' => Command::STANDARD_DELIVERY,
'choices' => array(
Command::STANDARD_DELIVERY => 'Standard',
Command::EXPRESS_DELIVERY => 'Express',
),
));
}