Django: How to write a GET query, using values_list attribute?

Question

I want to query a model, for single instance (eg., id=1), but I only want to return the value of a single field ('title'). I can achieve this with the following query:

SomeModel.objects.filter(pk=1).values_list('title', flat=True)

But a get is more efficient than a filter. However, values_list and values don't work with get queries. Ideally, I would prefer to do this:

SomeModel.objects.get(pk=1).values_list('title', flat=True)

But I get the following error: AttributeError: SomeModel has no attribute 'title'

What is the best way write this query?

You probably should not need worry about the perfomance between filter and get. What really matters is that you don't perform 1+N queries. Your current code almost looks like you are executing this query for various keys. If this is the case you should either perform a join or use pk__in=[1,2,3,4,5] to fetch the titles of more than one object at the same time. — bikeshedder
– bikeshedder, Commented Feb 2, 2013 at 2:07
Also OP doesn't need to worry about the performance difference between filter and get because there isn't any ;) — David Wolever
– David Wolever, Commented Feb 2, 2013 at 2:09
Unless, I suppose, OP is trying to work with multiple rows… In which case, get will be faster because it will throw an exception right away! — David Wolever
– David Wolever, Commented Feb 2, 2013 at 2:10

David Wolever · Accepted Answer · 2013-02-02 02:08:13Z

3

But a get is more efficient than a filter.

This is simply not true. A .get(…) will generate exactly the same SQL as a .filter(…).

Using:

SomeModel.objects.filter(pk=1)[0:1].values_list('title', flat=True)

Will do what you want, and will have exactly the same performance characteristics as .get(…) (actually, it will be a bit faster, because .get(…) checks to see if more than one row would be returned…):

In [4]: import logging

In [5]: l = logging.getLogger("django.db")

In [6]: l.setLevel(logging.DEBUG)

In [7]: User.objects.filter(id=1)[0:1].values_list()
DEBUG:django.db.backends:(0.006) SELECT "auth_user"."id", "auth_user"."username", "auth_user"."first_name", "auth_user"."last_name", "auth_user"."email", "auth_user"."password", "auth_user"."is_staff", "auth_user"."is_active", "auth_user"."is_superuser", "auth_user"."last_login", "auth_user"."date_joined" FROM "auth_user" WHERE "auth_user"."id" = 1  LIMIT 1; args=(1,)
Out[7]: [(1, u'admin', u'Admin', u'User', u'[email protected]', u'sha1$bf3bc$daa1fb58a8a41e15c730ae86bc0faf4c01fdd3a1', True, True, True, datetime.datetime(2013, 1, 8, 21, 15, 51, 855527), datetime.datetime(2012, 1, 10, 15, 13, 55))]

In [8]: User.objects.get(id=1)
DEBUG:django.db.backends:(0.001) SELECT "auth_user"."id", "auth_user"."username", "auth_user"."first_name", "auth_user"."last_name", "auth_user"."email", "auth_user"."password", "auth_user"."is_staff", "auth_user"."is_active", "auth_user"."is_superuser", "auth_user"."last_login", "auth_user"."date_joined" FROM "auth_user" WHERE "auth_user"."id" = 1 ; args=(1,)
Out[8]: <User: admin>

edited Feb 2, 2013 at 2:08

answered Feb 2, 2013 at 2:03

David Wolever

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1 Comment

bikeshedder Over a year ago

Django certainly does not use LIMIT for get. Otherwise it could never throw MultipleObjectsReturned. LIMIT 2 would be an option but this since you expect only one element to be returned this would just decrease the performance of the query.

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