title: How fill fields using a django forms.ModelChoiceField
body: <p>How I can obtain the Product Name when I select a opcion the Ref field from a django form.</p>
  
  <pre><code> class InvoiceForm(forms.Form):
      invoice= forms.CharField(widget = forms.TextInput())
      Ref= forms.ModelChoiceField(queryset=Producto.objects.all())
      Product = forms.CharField(widget = forms.TextInput())
  </code></pre>