title: In the Django admin, how can I filter down the options Django displays for a ManyToManyField that is part of the `fieldsets` property?
body: <p>Say I have models that looks like this:</p>
  
  <pre><code>class MyOtherModel(model.Model):
      name = CharField(max_length=50)
      type = CharField(max_length=5, default='A')
  
  class MyModel(models.Model):
      name = CharField(max_length=50)
      some_m2m = ManyToManyField('MyOtherModel')
  </code></pre>
  
  <p>and, in the admin, I want it to split up the <code>MyOtherModel</code> instances, based on type, to display in different parts of the screen, or with a different label, or whatever.</p>
  
  <p>I'd like to be able to do something like this (like you can do with the <code>list_display</code> property of a ModelAdmin):</p>
  
  <pre><code>class MyModelAdmin(admin.ModelAdmin):
      model = MyModel
      fieldsets = [
          (None, {
              'fields': [
                  ('name', 'some_m2m_type_A', 'some_m2m_type_B',)
              ]
          })
      ]
  
      def some_m2m_type_A(self):
          return MyOtherModel.objects.filter(type='A')
  
      def some_m2m_type_B(self):
          return MyOtherModel.objects.filter(type='B')
  </code></pre>
  
  <p>which, obviously, won't work, but you get the idea.</p>
  
  <p><br>
  I tried overriding <code>formfield_for_manytomany()</code>:</p>
  
  <pre><code>def formfield_for_manytomany(self, db_field, request=None, **kwargs):
      if db_field == 'some_m2m_type_A':
          kwargs["queryset"] = MyOtherModel.objects.filter(type='A')
      elif db_field == 'some_m2m_type_B':
          kwargs["queryset"] = MyOtherModel.objects.filter(type='B')
      return super(MyModelAdmin, self).formfield_for_manytomany(db_field, request, **kwargs)
  </code></pre>
  
  <p>similar to what is suggested in <a href="https://stackoverflow.com/questions/15228301/django-admin-how-to-customize-one-field-in-fieldsets">this question</a>, but Django still errors out with: </p>
  
  <blockquote>
    <p>FieldError: Unknown field(s) (some_m2m_type_A, some_m2m_type_B)
    specified for MyModel. Check fields/fieldsets/exclude attributes of class
    MyModelAdmin.</p>
  </blockquote>
  
  <p>and it errors out BEFORE it ever calls <code>formfield_for_manytomany()</code>...</p>
  
  <p>Is there a simple way to accomplish what I'm after in the Django Admin?  Or is it going to require a huge overhaul, overriding templates, and other hard-to-maintain hackery?</p>