Question

I’m using Django 1.3 with Postgresql 8.4 and I’ve got models like the following (irrelevant stuff removed):

class Service(models.Model):
    name = models.CharField(max_length=80)

class Location(models.Model):
    name = models.CharField(max_length=80)
    services = models.ManyToManyField(Service, through='LocalService')

class LocalService(models.Model):
    location = models.ForeignKey(Location)
    service = models.ForeignKey(Service)

I’m trying to get a distinct set of Service objects, filtered by attributes of linked Location objects, and ordered randomly. I tried this first:

Service.objects.filter(location__name__icontains='o').distinct().order_by('?')

…but that throws this exception:

DatabaseError: for SELECT DISTINCT, ORDER BY expressions must appear
in select list

After Googling around, I discovered that to achieve this kind of result at the SQL level, you need to put the DISTINCT and the ORDER BY in separate query levels, ie: use a subquery. If I subquery a DISTINCT set of results I can randomly order them like this:

SELECT * 
FROM (
    SELECT DISTINCT s.*
    FROM profile_service s
    JOIN profile_localservice ls
    ON ls.service_id = s.id
    JOIN profile_location l
    ON ls.location_id = l.id
    WHERE l.name LIKE '%o%'
) as temptable
ORDER BY RANDOM()

Do I need to use the Manager.raw() method with this SQL query to get my set of model instances, or is there a simpler way to do this from within the Django API?

Answer 1

Depending on your exact requirements the following might work (and potentially perform better than ORDER BY Random()). I’m not sure about Postgresql, but with MySQL randomized ordering on anything but a tiny dataset is really slow.

services = list(Service.objects.filter(location__name__icontains='o').distinct())
random.shuffle(services)