title: Editing json_array field with symfony form
body: <p>In order to edit a json_array symfony field type of an entity, I am trying to convert it to an editable string with a text area in a form.</p>
  
  <p>I have created a DataTransformer which is making the conversion <em>JSONString &lt;-> array</em>:</p>
  
  <pre><code>/**
   * Transform an array to a JSON string
   */
  public function transform($array)
  {
      return json_encode($array);
  }
  
  /**
   * Transform a JSON string to an array
   */
  public function reverseTransform($string)
  {
      return json_decode($string, true);
  }
  </code></pre>
  
  <p>When I create my form with the form builder I am able to convert the array to string like that:</p>
  
  <pre><code>$builder-&gt;add($builder-&gt;create('info', 'textarea')-&gt;addModelTransformer(new ArrayToJSONStringTransformer()))
  </code></pre>
  
  <p>But when I submit the form, Symfony is creating a new entity and this field is converted as an empty array.</p>
  
  <p>How should I do ?</p>