title: Django nested inlines not working?
body: <p>I've installed django-nested-inline but I have problems. Here's my code:</p>
  
  <pre><code>from django.contrib import admin
  from nested_inline.admin import NestedStackedInline, NestedModelAdmin
  from .models import Positive, Negative, Option, Question
  
  class PositiveInline(NestedStackedInline):
      model = Positive
  
  class NegativeInline(NestedStackedInline):
      model = Negative
  
  class OptionInline(NestedStackedInline):
      model = Option
      inlines = [PositiveInline, NegativeInline,]
  
  class QuestionAdmin(NestedModelAdmin):
      fieldsets = [
          (None,               {'fields': ['question_title']}),
          ('Date information', {'fields': ['pub_date'], 'classes': ['collapse']}),
      ]
      inlines = [OptionInline,]
  
  admin.site.register(Question, QuestionAdmin)
  </code></pre>
  
  <p>When I try to add a new Question, I have this error:</p>
  
  <pre><code>AttributeError at /admin/question/question/add/
  
  'OptionInline' object has no attribute 'queryset'
  </code></pre>
  
  <p>Am I doing something wrong?</p>