title: Django Admin - Overriding the widget of a custom form field
body: <p>I have a custom TagField form field.</p>
  
  <pre><code>class TagField(forms.CharField):
      def __init__(self, *args, **kwargs):
          super(TagField, self).__init__(*args, **kwargs)
          self.widget = forms.TextInput(attrs={'class':'tag_field'})
  </code></pre>
  
  <p>As seen above, it uses a TextInput form field widget. But in admin I would like it to be displayed using Textarea widget. For this, there is <code>formfield_overrides</code> hook but it does not work for this case.</p>
  
  <p>The admin declaration is:</p>
  
  <pre><code>class ProductAdmin(admin.ModelAdmin):
      ...
      formfield_overrides = {
          TagField: {'widget': admin.widgets.AdminTextareaWidget},
      }
  </code></pre>
  
  <p>This has no effect on the form field widget and <code>tags</code> are still rendered with a TextInput widget.</p>
  
  <p>Any help is much appreciated.</p>
  
  <p>--<br>omat</p>