title: Add element to doctrine ArrayCollection by Id
body: <p>I find very annoying to have to fetch an object by id from database every time I want to add it to a relationship. Is there a way to add an object to a a relationship by id instead of adding the whole object?</p>
  
  <p>This is my actual code:</p>
  
  <pre><code>...
  $person = $em-&gt;getRepository("Person")-&gt;findOneById($id);
  $me-&gt;getPersons()-&gt;add($person);
  ...
  </code></pre>
  
  <p>I would like to have something like this:</p>
  
  <pre><code>...
  $me-&gt;getPersons()-&gt;add($id);
  ...
  </code></pre>
  
  <p>Then I would be saving one trip to the database! Which I like better! Is it possible?</p>