title: Custom label on ModelChoiceField form field
body: <p>I am using forms with the following:</p>
  
  <pre><code>class InvoiceModelForm ( forms.ModelForm ):
  
      u = forms.ModelChoiceField ( queryset = User.objects.all () )
  </code></pre>
  
  <p>But in the form field it is displaying the username.  I would like to change that to use the first and last names. </p>
  
  <p>I do not want to change the</p>
  
  <pre><code>def __str__(self):
      return self.username
  </code></pre>
  
  <p>How can I change what values that are displayed in the form field?</p>