title: Twig and Symfony2 - Entity was not found
body: <p>I have an entity that is related to some other entities.
  On the end, I have an object like tat:</p>
  
  <pre><code>paper.submission.authors
  </code></pre>
  
  <p>For some of the paper.submission, there is no author, and in my twig template, I am doing:</p>
  
  <pre><code>{% for author in paper.submission.authors}
      do something
  {% endfor %}
  </code></pre>
  
  <p>And for the paper.submission with no authors, I am getting "Entity was not found" exception.</p>
  
  <p>Is thee any possibility to test if the object exists before my for loop.</p>
  
  <p>I have try the is defined, it is always true. Then, I have tryed is not null, but this is also generating the exception.</p>
  
  <p>Thank you very much in advance.</p>