How to pull a random record using Django's ORM?

Question

I have a model that represents paintings I present on my site. On the main webpage I'd like to show some of them: newest, one that was not visited for most time, most popular one and a random one.

I'm using Django 1.0.2.

While first 3 of them are easy to pull using django models, last one (random) causes me some trouble. I can ofc code it in my view, to something like this:

number_of_records = models.Painting.objects.count()
random_index = int(random.random()*number_of_records)+1
random_paint = models.Painting.get(pk = random_index)

It doesn't look like something I'd like to have in my view tho - this is entirely part of database abstraction and should be in the model. Also, here I need to take care of removed records (then number of all records won't cover me all the possible key values) and probably lots of other things.

Any other options how I can do it, preferably somehow inside the model abstraction?

Answer 1

You could create a manager on your model to do this sort of thing. To first understand what a manager is, the Painting.objects method is a manager that contains all(), filter(), get(), etc. Creating your own manager allows you to pre-filter results and have all these same methods, as well as your own custom methods, work on the results.

EDIT: I modified my code to reflect the order_by['?'] method. Note that the manager returns an unlimited number of random models. Because of this I've included a bit of usage code to show how to get just a single model.

from django.db import models

class RandomManager(models.Manager):
    def get_query_set(self):
        return super(RandomManager, self).get_query_set().order_by('?')

class Painting(models.Model):
    title = models.CharField(max_length=100)
    author = models.CharField(max_length=50)

    objects = models.Manager() # The default manager.
    randoms = RandomManager() # The random-specific manager.

Usage

random_painting = Painting.randoms.all()[0]

Lastly, you can have many managers on your models, so feel free to create a LeastViewsManager() or MostPopularManager().

Answer 2

Simply use:

MyModel.objects.order_by('?')[0]

It is documented in QuerySet API.

Answer 3

You could simply do:

models.Painting.objects.all().order_by('?')[:1].get()

which is much more efficient.

Answer 4

The solutions with order_by('?')[:N] are extremely slow even for medium-sized tables if you use MySQL (don't know about other databases).

order_by('?')[:N] will be translated to SELECT ... FROM ... WHERE ... ORDER BY RAND() LIMIT N query.

It means that for every row in table the RAND() function will be executed, then the whole table will be sorted according to value of this function and then first N records will be returned. If your tables are small, this is fine. But in most cases this is a very slow query.

I wrote simple function that works even if id's have holes (some rows where deleted):

def get_random_item(model, max_id=None):
    if max_id is None:
        max_id = model.objects.aggregate(Max('id')).values()[0]
    min_id = math.ceil(max_id*random.random())
    return model.objects.filter(id__gte=min_id)[0]

It is faster than order_by('?') in almost all cases.

Answer 5

Using order_by('?') will kill the db server on the second day in production. A better way is something like what is described here: Getting a random row from a relational database.

class PaintingManager(models.Manager):
    def random(self):
        count = self.aggregate(count=Count('id'))['count']
        random_index = randint(0, count - 1)
        return self.all()[random_index]