title: Symfony collection forms - Save to json_array
body: <p>Hove to create collections of field and store in one db column type json_array?</p>
  
  <p>My entity have column date_data which is json_array type. I want to render two fields on frontent.</p>
  
  <p>First Field -> from - date type. <br>
  Second Field -> to - date type.</p>
  
  <p>I use jQuery repeater lib, for render this fields as repeater field on frontend. And want to store fields data from repeater in date_data column in db like this.</p>
  
  <p><code>[{"from": '12/31/2009' , "to": '01/16/2010' }, {"from": '02/10/2011' , "to": '02/16/2011' }]</code></p>