title: relation between two form fields
body: <p>I'm new in django. I need somthing like this in my form.py:</p>
  
  <pre><code>class SearchForm(forms.Form):
      CHOICES1 = (('a','a'),
                 ('b','b'),)
      field1 = forms.ChoiceField(choices=CHOICES1)
  #if field1=='a':
      CHOICES2 = (('c','c'),
                 ('d','d'),)
      field2 = forms.ChoiceField(choices=CHOICES2) 
  #if field1=='b':
      CHOICES2 = (('e','e'),
                 ('f','f'),)
      field2 = forms.ChoiceField(choices=CHOICES2)
  </code></pre>
  
  <p>but I have no idea how should I do this.</p>