Getting random object of a model with django-rest-framework - django

Question

In my Django project I need to provide a view to get random object from a model using django-rest-framework. I had this ListAPIView:
class RandomObject(generics.ListAPIView):
queryset = MyModel.objects.all().order_by('?')[:1]
serializer_class = MyModelSerializer
...
It worked fine but order_by('?') takes a lot of time when launched on big database. So I decided to use usual Python random.
import random
def pick_random_object():
return random.randrange(1, MyModel.objects.all().count() + 1)
class RandomObject(generics.ListAPIView):
queryset = MyModel.objects.all().filter(id = pick_random_object())
...
I found out a strange thing when tried to use this. I launched Django development server and sent some GET requests, but I got absolutely the same object for all of the requests. When dev server restarted and another set of requests sent I'm getting another object, but still absolutely the same one for all of requests - even if random.seed() was used first of all. Meanwhile, when I tried to get a random object not via REST but via python manage.py shell I got random objects for every time I called pick_random_object().
So everything looks good when using shell and the behavior is strange when using REST, and I have no clue of what's wrong.
Everything was executed on Django development server (python manage.py runserver).

Answer 1

As #CarltonGibson noticed, queryset is an attribute of RandomObject class. Hence it cached and cannot be changed any later. So if you want to make some changeable queryset (like getting random objects at every request) in some APIView, you must override a get_queryset() method. So instead of
class RandomObject(generics.ListAPIView):
queryset = MyModel.objects.all().filter(id = pick_random_object())
...
you should write something like this:
class RandomObject(generics.ListAPIView):
#queryset = MyModel.objects.all().filter(id = pick_random_object())
def get_queryset(self):
return MyModel.objects.all().filter(id = pick_random_object())
Here pick_random_object() is a method to get random id from the model.

Answer 2

Since it's an attribute of the class, your queryset is getting evaluated and cached when the class is loaded, i.e. when you start the dev server.
I'd try pulling a list of primary keys, using values_list() — the flat=True example does exactly what you need. Ideally cache that. Pick a primary key at random and then use that to get() the actual object when you need it.
So, how would that go?
I'd define a method on the view. If you forget the caching, the implementation might go like this:
# Lets use this...
from random import choice
def random_MyModel(self):
"""Method of RandomObject to pick random MyModel"""
pks = MyModel.objects.values_list('pk', flat=True).order_by('id')
random_pk = choice(pks)
return MyModel.objects.get(pk=random_pk)
You might then want to cache the first look up here. The caching docs linked above explain how to do that. If you do cache the result look into the db.models signals to know when to invalidate — I guess you'd post_save, checking the created flag, and post_delete.
I hope that helps.