title: Django admin overwriting fieldsets
body: <p>Suppose we have one class in the model from which several other classes are inherited. Now we define also an admin class to control the layout in the admin interface. </p>
  
  <p>We can make all other admin classes inherite (just as we did in the model), but how can we overwrite the fieldsets to add some new values?</p>
  
  <p>Example:</p>
  
  <p>in model file:</p>
  
  <pre><code>class A(models.Model):
    field1 = models.TextField(..)
    field2 = models.TextField(..)
  
  class B(A):
    field3 = models.TextField(..)
  </code></pre>
  
  <p>in admin file:</p>
  
  <pre><code>class A_Admin(admin.ModelAdmin):
      fieldsets = (
          (None, {
              'fields': (( 'field1', 'field2'))
          }),
      )
  
  class B_Admin(A_Admin):
      pass # here I notice that it takes the fields from A_admin, I would like to add my field 3 without rewriting the fieldsets 
  
  admin.site.register(A, A_Admin)
  admin.site.register(B, B_Admin)
  </code></pre>
  
  <p>the resulting fieldsets would be :</p>
  
  <pre><code>(
          (None, {
              'fields': (( 'field1', 'field2'))
          }),
  
          ('Specific to B', {
              'fields': ('field3')
          }),
      )
  </code></pre>