title: Django admin choice field
body: <p>I have a model that has a CharField and in the admin I want to add choices to the widget. The reason for this is I'm using a proxy model and there are a bunch of models that share this CharField but they each have different choices.</p>
  
  <pre><code>class MyModel(MyBaseModel):
      stuff = models.CharField('Stuff', max_length=255, default=None)
  
      class Meta:
          proxy = True
  
  class MyModelAdmin(admin.ModelAdmin):
      fields = ('stuff',)
      list_display = ('stuff',)
  admin.site.register(MyModel, MyModelAdmin)
  </code></pre>
  
  <p>For this model I want to use <code>MY_CHOICES</code> in <code>MyModelAdmin</code>.</p>
  
  <p>Do I override a widget? Do I need to override the whole form?</p>