title: Association in Symfony2 entity on field other than id does not display correctly in form
body: <p>In an entity in my Symfony app there is an association that is not based on id's, but on serial numbers. The association looks like this:</p>
  
  <pre><code>/**
   * @var Department
   *
   * @ORM\ManyToOne(targetEntity="Department")
   * @ORM\JoinColumn(name="department_serial", referencedColumnName="serial")
   */
  protected $department;
  </code></pre>
  
  <p>When I create a form for this entity, I add the field Department like so:</p>
  
  <pre><code>-&gt;add('department', null, array
  (
    'label' =&gt; 'Choose department',
    'required' =&gt; true,
    //some more...
  ))
  </code></pre>
  
  <p>The problem is that when the form is rendered I end up with wrong html list item values. The items seem to get auto numbering, as the values are not the actual department id's. Neither do they correspond to the serial numbers, which is my goal. This is an example of what I get:</p>
  
  <pre><code>&lt;select id="someId" name="form[someName]"&gt;
    &lt;option value="0"&gt;Department A&lt;/option&gt;
    &lt;option value="1"&gt;Department B&lt;/option&gt;
    &lt;option value="2"&gt;Department C&lt;/option&gt;
    &lt;option value="3"&gt;Department D&lt;/option&gt;
    &lt;option value="4"&gt;Department E&lt;/option&gt;
    &lt;option value="5"&gt;Department F&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>The values of the list items above, should have been the serial number of each department. Since they are not, the link between the entity and its department seems lost and saving my form is not possible. (Choosing a department is required and departments with these serial numbers [0, 1, 2, 3, 4, 5] do not exist.)</p>
  
  <p>Hence, my question is: how can I indicate in my form type which property of the associated entity should be used as the value in the html list? I know that there is an option called "property" to change the label of each list item, but is there a way of changing its value?</p>