Question

I have two models like this:

class Store(models.Model):
    name = models.CharField(max_length=255)

class Order(models.Model):
    store = models.ForeignKey(Store)
    date = models.DateTimeField(auto_now_add=True)
    success = models.BooleanField()

I would like to filter all the records from the Store model whose latest order was successful i.e. success == True.

Although it looks very simple, I’m having issues figuring how I would accomplish this using the ORM query system.

Any help? Thanks.

Answer 1

My approach is this: do 2 lists, first one with (id_store, last_success_date) tuples and second one with (id_store, last_date) tuples:

l_succ = stores.objects.filter( 
                       order__success = True 
                  ).annotate(
                       last_success=Max('order__date')
                  ).value_list (
                       'id', 'last_success'
                  )
#l_succ = [ (1, '1/1/2011'), (2, '31/12/2010'), ... ] <-l_succ result

l_last = stores.objects.annotate(
                       last_date=Max('order__date')
                  ).value_list (
                       'id', 'last_date'
                  )
#l_last = [ (1, '1/1/2011'), (2, '3/1/2011'), ... ]   <-l_last result

Then take store ids for stores that last data and last success date are equals, and you have the query:

store_success_ids =  [ k[0] for k in l_succ if k in l_last ]
#store_success_ids = [1, 5, ... ]          <-store_success_ids result
#Cast l_last to dictionary to do lookups if you have a lot of stores.

result = Store.objects.filter( pk__in = store_success_ids)        

It seems an elegant solution, only four lines of code for a complex query (but with a simple requeriment). Disclaimer, it is not tested.