title: Display value in Charfield for foreign key in django on error
body: <p>Let's say I've got a model and it has a foreign key to another one.</p>
  
  <pre><code>class ModelA(models.Model):
      field = models.CharField(max_length=100)
  
  class ModelB(models.Model):
      model_a = models.ForeignKey(ModelA)
  </code></pre>
  
  <p>Than I've got this form:</p>
  
  <pre><code>class FormB(models.ModelForm):
      model_a = forms.CharField(required=True)
  
      def clean(self):
          model_a = self.cleaned_data["model_a"]
          try:
              v = ModelA.objects.get(model_a=model_a)
              self.cleaned_data["model_a"] = v
          except Exception:
              self._errors['model_a'] = ErrorList(["ModelA not found"])
      return self.cleaned_data
  </code></pre>
  
  <p>Now, whenever I enter a char value in FormB, it'll search for it in the ModelA and
  return the cleaned data.</p>
  
  <p>When I use the form to list the pre-existing instance it shows the ID and not the value.</p>
  
  <pre><code>def my_view(request):
      instance = ModelB.objects.get()[0]
      form = FormB(instance=instance)
      return render_to_response("bla.html", {"form" : form})
  </code></pre>
  
  <p>Does anybody knows how I could show the value in this CharField when I pass the instance?</p>
  
  <p>Thanks, 
  Nico</p>