title: What is a PersistentCollection?
body: <p>I have a One-To-Many relationship in my database. The <strong>editing, deleting and adding</strong> of elements used to work just fine. However, after a few changes (I can't track them all back), it doesn't work anymore.<br>
  Now, when I fetch the object with the One-To-Many relationship, I get a Persistent Collection to represent my collection of the Many-side. I suppose that wasn't the case before. In my constructor, I create a new Array Collection and not a Persistent Collection.</p>
  
  <p>I have looked up the doctrine documentation to find that: </p>
  
  <blockquote>
    <p>A PersistentCollection represents a collection of elements that have
    persistent state.</p>
  </blockquote>
  
  <p>I do not understand what that means.</p>
  
  <p>Could you please tell me:</p>
  
  <ol>
  <li>Is it normal to have Persistent Collections with a simple One-To-Many?   </li>
  <li>How do those collections may appear? (rather than the
      usual ArrayCollection) </li>
  <li>Finally, what exactly is the use of a
          Persistent Collection?</li>
  </ol>