title: Add and Remove action on Symfony2 controller
body: <p>I'm a newbie on Symfony and I'm working at a web application which support a mobile app.
  I need to build a grocery list of food products.
  Products can be searched by name and if I'm interested in I can add/remove them to a shopping list with increasing unit and "+" or "-" .
  I have <code>Product</code> and <code>ShoppingList</code> entities, but I have no idea how to build these functions.</p>