title: Sf2 split one formType in many form on one page
body: <p>Here is what i would like to achieve. (i thought it would be easier).
  I have a UserEntity.
  Each field of this entity have an :
   "@Assert\NotNull( groups={"step-1"} )" 
  or 
   "@Assert\NotNull( groups={"step-2"} )"</p>
  
  <pre><code>/** @ORM\Column(name="username", type="string", length=255, nullable = true)     
  * @Assert\NotNull(groups={"step-1"}) 
  */
  protected $username;
  
  
  /** @ORM\Column(name="city", type="string", length=255, nullable = true)     
  * @Assert\NotNull( groups={"step-2"} ) 
  */
  protected $city;
  </code></pre>
  
  <p>On the formType, i have my 2 fields plus 2 submit buttons : </p>
  
  <pre><code>-&gt;add('username')
  -&gt;add('city')
   -&gt;add('submit_form_1', SubmitType::class, array(
              'validation_groups' =&gt; 'step-1',
              )
          )
  -&gt;add('submit_form_2', SubmitType::class, array(
              'validation_groups' =&gt; 'step-2',
              )
          )
  </code></pre>
  
  <p>What i would like to do is display the Username inputType,followed by the submitOne button.
  Then display the City inputType, followed by the submitTwo button.</p>
  
  <p>Then , when the user click on submit One, validate the datas, and save persist this field in the database.</p>
  
  <p>Here is my view : </p>
  
  <pre><code>{{ form_start(form, { 'action': path('fos_user_profile_edit'), 'attr': { 'novalidate' : 'novalidate', 'class': 'fos_user_profile_edit' } }) }}
  {{ form_row(form.userType.city) }}
  {{ form_widget(form._token) }}
  {{ form_widget(form.userType.submit_form_2) }}
  {{ form_end(form, {'render_rest': false}) }}
  
  {{ form_start(form, { 'action': path('fos_user_profile_edit'), 'attr': { 'novalidate' : 'novalidate', 'class': 'fos_user_profile_edit' } }) }}
  {{ form_row(form.userType.username) }}
  {{ form_widget(form._token) }}
  {{ form_widget(form.userType.submit_form_1) }}
  {{ form_end(form, {'render_rest': false}) }}
  </code></pre>
  
  <p>When i submit the first form everything works great.
  When i submit the second one, i've got a "Invalid CSRF token error" and indeed if i look at the dom, only the first form have an input type hidden with the CSRF token.</p>
  
  <p>I did try to input manually the CSRF token (just as a test), and when i submit a form, the datas of this form are persisted, but the the data form the other form are set to null.</p>
  
  <p>I'm kind of stuck right now.
  Maybe i'm doing this wrong, the general idea is to split a single entity into differents forms on a same page.</p>
  
  <p>Thanks !</p>