title: django model form inheritance
body: <p>I'm using django modelform inheritence in my modelform but it seems to be not working here is my code sample </p>
  
  <pre><code>class ArticleForm(forms.ModelForm):
    title = forms.CharField(required=True)
    sites = forms.ModelMultipleChoiceField(required=True, queryset= Sites.objects.all().order_by('name'), widget=forms.SelectMultiple())
  
  class ArticleAddForm(ArticleForm):
     class Meta(ArticleForm.Meta):
         exclude = ('sites',)
  </code></pre>
  
  <p>i want to exclude "sites" from "ArticleAddForm" but while validating it is raising form validation error sites field required please help?</p>