title: How do you update a Django Form Meta class fields dynamically from the form constructor?
body: <p>I want to update the <code>Meta.fields</code> dynamically.  Is it possible to do it from the Form constructor?  I tried the following but <code>year</code> doesn't show up during the form generation.  Only <code>name</code> and <code>title</code> are displayed.</p>
  
  <pre><code>class Author(models.Model):
      name = ...
      title = ...
      year = ...
  
  class PartialAuthorForm(ModelForm):
      class Meta:
          model = Author
          fields = ('name', 'title')
  
      def __init__(self, *args, **kwargs):
          self.Meta.fields += ('year',)
  </code></pre>