title: How to create a collection of several fields without any custom Form Type class?
body: <p>I have lots of collections to create, and don't want to create a FormType for each entry_type, because they are used only once. </p>
  
  <p>So instead of giving a form type FQCN in the <code>entry_type</code> option, I tried to put a fresh <code>Type</code> directly coming from the form builder:</p>
  
  <pre><code>$type = $this
     -&gt;get('form.factory')
     -&gt;createBuilder(Type\FormType::class)
     -&gt;add('label', Type\TextType::class, [
         'label' =&gt; 'Key',
     ])
     -&gt;add('value', Type\TextType::class, [
         'label' =&gt; 'Value',
     ])
     -&gt;getType()
  ;
  
  $form = $this
     -&gt;get('form.factory')
     -&gt;createBuilder(Type\FormType::class)
     -&gt;add('hash', Type\CollectionType::class, [
         'entry_type'    =&gt; $type,
         'entry_options' =&gt; [],
         'allow_add'     =&gt; true,
         'allow_delete'  =&gt; true,
         'prototype'     =&gt; true,
         'required'      =&gt; false,
         'delete_empty'  =&gt; true,
     ])
     -&gt;getForm()
  ;
  </code></pre>
  
  <p>But for some reasons, the prototype is invalid:</p>
  
  <pre><code>&lt;div class="form-group"&gt;
      &lt;label class="control-label required"&gt;__name__label__&lt;/label&gt;
      &lt;div id="form_hash___name__"&gt;&lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>All child fields from my manually-created <code>$type</code> are missing. Where is my mistake?</p>