title: Problems with updating composite keys in sonata admin bundle
body: <p>I am new to sonata admin bundle</p>
  
  <p>I have created a form in sonata admin bundle. </p>
  
  <pre><code> protected function configureFormFields(FormMapper $formMapper)
     {
  
      $formMapper
      -&gt;with('General')
  
      -&gt;add('userid', null, array('label' =&gt; 'User'))
      -&gt;add('cityid', null, array('label' =&gt; 'City Name'))
  
      -&gt;end();    
     }
  </code></pre>
  
  <p>Here <code>userid</code> and <code>cityid</code> are composite key.</p>
  
  <p>I am able to create a new record successfully. But Updation  on the same record by changing any one of the composite key creates a problem. </p>
  
  <p>The record is updated successfully in the database but it throws the exception</p>
  
  <pre><code>unable to find the object with id : 1~1 
  </code></pre>
  
  <p>where 1~1 is the id of user and city prior its updation. How do i resolve this Exception?</p>
  
  <p>Thanks in advance.</p>