title: Adding a row for the sum of a field to Django Admin
body: <p>I have an invoices app I would like to be able to display the total of the invoice in the admin panel at the bottom. (I'm using Django-suit)</p>
  
  <p><img src="https://i.stack.imgur.com/gDBMF.png" alt="enter image description here"></p>
  
  <p>Is there an easy way of doing this within Django?</p>
  
  <p>e: Should something like this work?</p>
  
  <pre><code>class InvoiceAdmin(admin.ModelAdmin):
      inlines = [InvoiceItemInline]
      list_display = ('description', 'date', 'status', 'invoice_amount')
  
      def invoice_amount(self, request):
          amount = InvoiceItem.objects.filter(invoice__id=request.id).values_list('price', flat=True)
          quantity = InvoiceItem.objects.filter(invoice__id=request.id).values_list('quantity', flat=True)
          total_current = amount(float) * quantity(float)
          total_amount = sum(total_current)
          return total_amount
  </code></pre>