Nested forms in symfony empty after form submit - symfony

Question

I have a question related to nested forms in symfony.
I have this form
{{ form_start(form, {'attr': {'class': 'needs-validation'}} ) }}
....
<div id="slot-fields-list" data-prototype="
{% filter escape %}
{% include 'slot/form/prototype.html.twig' with {'form': form.slot.vars.prototype} %}
{% endfilter %}"
data-widget-tags="{{ '<span></span>'|e }}">
</div>
{{ form_end(form) }}
.......................................
->add('slot', CollectionType::class, [
'entry_type' => SlotType::class,
'entry_options' => ['label' => false],
'label' => false,
'allow_add' => true,
'allow_delete' => true,
'error_bubbling' => false
])
Work fine, I have data in data-prototype after render template. When I submit the form and if the form has erreurs this nested form is added after submit button and data-prototype is empty. Any ideas ? Thx in advance

Answer 1

The documentation for CollectionType is actually quite good, but perhaps some particular details need reiteration.
First the documentation tells you, how to render the collection items (your slots probably):
<ul class="tags">
{# iterate over each existing tag and render its only field: name #}
{% for tag in form.tags %}
<li>{{ form_row(tag.name) }}</li>
{% endfor %}
</ul>
This is what's missing in your case. Just to avoid confusion, their <ul class="tags"> is your <div id="slot-fields-list" ...>. You have to go through the form.slot with
{% for slot in form.slot %}
{{ form_row(slot) }} <!--- or render your sub-form properly -->
{% endfor %}
now. Symfony is very reluctant to omit form elements. So when you place a {{ form_end(form) }} it will render the rest of the form elements, which have not been rendered before, to your form, before closing the <form>. (the same behaviour can be achieved by {{ form_rest(form) }}, if you ever need it)
That is the reason, why your subforms will appear after your submit button and completely out of place - because they weren't rendered anywhere before.
However, this does not yet explain the missing data-prototype, but perhaps there is a simple explanation as well ...
you possibly already know about the form themes, where you could add your own slot_widget, slot_row, slot_label and have symfony do all that weird include-stuff for you.