title: Filter by no field in Django Admin
body: <p>How to resolve this?</p>
  
  <p>model.py</p>
  
  <pre><code>class Sale(models.Model):
      customer = models.ForeignKey(Customer, related_name='customer_sale')
  
  ...
  
      def get_itens(self):
          return self.sales_det.count()
      item = property(get_itens)
  
  class SaleDetail(models.Model):
      sale = models.ForeignKey(Sale, related_name='sales_det')
      product = models.ForeignKey(Product, related_name='product_det')
      quantity = models.PositiveSmallIntegerField()
  </code></pre>
  
  <p>admin.py</p>
  
  <pre><code>class SaleAdmin(admin.ModelAdmin):
      list_display = ('__unicode__', 'customer', 'get_itens', 'get_total')
      readonly_fields = ['get_total']
      list_filter = ('customer','item')
  </code></pre>
  
  <h1>Question</h1>
  
  <p>How to filter by item in this case?
  Because i dont filter if no field or calculated field or field created with property.</p>
  
  <p>Error message trace back</p>
  
  <pre><code>ERRORS:
  &lt;class 'vendas_project.vendas.admin.SaleAdmin'&gt;: (admin.E116) The value of 'list_filter[1]' refers to 'item', which does not refer to a Field.
  </code></pre>