title: Symfony - How to add form fields that are not embeeded in the underlying object
body: <p>I'm new with Symfony and I'm trying to understand how to make a good form for a login. 
  As far as I'm now, I understood that when i'm calling the "isValid()" method, I'm checking if the form submitted data is consistent with the properties of the underlying object (i.e. login object).</p>
  
  <p>Now, this implies an absolute equality between form field and object properties, which is the core of my problem: </p>
  
  <p>what if I want to use fields that not match with any of the login object properties?
  E.g. "remember my login" checkbox, which automates login for already logged user.</p>
  
  <p>Thanks</p>