title: Disable choice list in Django admin, only for editing
body: <p>I want to disable some fields when I am editing an object. I have managed to do this for text fields, but it's been impossible for a dropdown list (choice list).</p>
  
  <p>I am doing this action in the constructor of the form.</p>
  
  <pre><code>class OrderModelForm(forms.ModelForm):
      def __init__(self, *args, **kwargs):
          super(forms.ModelForm, self).__init__(*args, **kwargs)
          instance = getattr(self, 'instance', None)
          if instance and instance.pk:
              self.fields['description'].widget.attrs['readonly'] = True
              self.fields['city_code'].widget.attrs['disabled'] = True
  </code></pre>
  
  <p>Notice how I made it for both with different <strong>keywords</strong>, but I can't do it for my <code>customer_id</code> field.</p>