title: symfony render json_array entity type and save using form
body: <p>Imagine I have an Article entity.<br>
  And in this entity have a report attribute which is an json_array type.</p>
  
  <p>Json_array's data may like <br>
  {"key1":"value1","key2":{"k1":"v1","k2","v2"...},"key3":["v1","v2","v3"...]...}.<br>
  I mean the json_array may contains simple key:value<br> or the value may also contains key:vaule <br>or the value may an array.<br></p>
  
  <p>Now I don't know how to use symfony form to render and save these json_array like other normal attribute(e.g.,title).<br>At the same time,I want to manage the key label name with an meaning name just like change the title field's label.<br>
  How to achieve this,I feel very difficult.</p>
  
  <pre><code>class Article
  {
      /**
       * @var int
       *
       * @ORM\Column(name="id", type="integer")
       * @ORM\Id
       * @ORM\GeneratedValue(strategy="AUTO")
       */
      private $id;
  
      /**
       * @var string
       *
       * @ORM\Column(name="title", type="string", length=255)
       */
      private $title;
  
      /**
       * @var array
       *
       * @ORM\Column(name="report", type="json_array")
       */
      private $report;
  
  }
  </code></pre>