Django admin: how to sort by one of the custom list_display fields that has no database field

I loved Greg's solution to this problem, but I'd like to point that you can do the same thing directly in the admin:

from django.db import models

class CustomerAdmin(admin.ModelAdmin):
    list_display = ('number_of_orders',)

    def get_queryset(self, request):
    # def queryset(self, request): # For Django <1.6
        qs = super(CustomerAdmin, self).get_queryset(request)
        # qs = super(CustomerAdmin, self).queryset(request) # For Django <1.6
        qs = qs.annotate(models.Count('order'))
        return qs

    def number_of_orders(self, obj):
        return obj.order__count
    number_of_orders.admin_order_field = 'order__count'

This way you only annotate inside the admin interface. Not with every query that you do.

I haven't tested this out (I'd be interested to know if it works) but what about defining a custom manager for Customer which includes the number of orders aggregated, and then setting admin_order_field to that aggregate, ie

from django.db import models 


class CustomerManager(models.Manager):
    def get_query_set(self):
        return super(CustomerManager, self).get_query_set().annotate(models.Count('order'))

class Customer(models.Model):
    foo = models.CharField[...]

    objects = CustomerManager()

    def number_of_orders(self):
        return u'%s' % Order.objects.filter(customer=self).count()
    number_of_orders.admin_order_field = 'order__count'

EDIT: I've just tested this idea and it works perfectly - no django admin subclassing required!