Date lookups in Django

Published on Oct. 10, 2015, 2:27 p.m.

database django lookups orm postgres python

A while ago I tweeted out something that I've wanted to see in Django for a very long time, yet have never really taken the time to investigate or implement it:

I wish #django had this: M.objects.filter(datetimefield__date=http://t.co/MVFXsN4Ivk(2015, 6, 29)) Has that ever been attempted?

— Brad Montgomery (@bkmontgomery) June 29, 2015

Django's ORM has a very rich set of field lookups, but at present, it doesn't support an exact date lookup. At least not with the syntax I would expect. Luckily, that tweet got some very handy responses, so let's do a bit of exploring.

Assume we have the following model. For the now, the only field we care about is the created field.

from django.db import models

class Item(models.Model):
    title = models.CharField(max_length=128)
    created = models.DateTimeField(auto_now_add=True)

Again, what I wish we could do is an exact date lookup using a __date lookup. If you try that:

from datetime import date
Item.objects.filter(created__date=date(2015, 10, 1))

Well, you get an exception.

FieldError: Unsupported lookup 'date' for AutoCreatedField or join on the field not permitted.

Fear not, there are a handful of ways to acheive the same results. One way is to use the date-based lookups that django already provides. Such as __day, __month, and __year.

dt = date(2015, 10, 1)
Item.objects.filter(
    created__day=dt.day, 
    created__month=dt.month, 
    created__year=dt.year
)

That's a technique I've used quite a lot in the past. It's fairly verbose, yet it's pretty readable. Another technique is to use the __range lookup. You can use a datetime object's replace method to set minimum and maximum values for the time-related attributes (and the docs for datetime objects list these values).

# Create a datetime object spanning a full day
dt = datetime.now()
start = dt.replace(hour=0, minute=0, second=0, microsecond=0)
end = dt.replace(hour=23, minute=59, second=59, microsecond=999999)

# Query for objects that fall within that day.
Item.objects.filter(created__range=(start, end))

One neat suggestion came from Jason Myers notes that you can use the __contains lookup with dates:

Item.objects.filter(created__contains=date(2015, 10, 1))

Another from Josh Ourisman takes advantage of the fact that dates are really just strings (at least in postgres):

Item.objects.filter(created__startswith='2015-10-01')

Joshua Ginsberg suggested using a custom database function prior to filtering. So, in the spirit of learning new things available in Django 1.8, I did just that. With a Func() expression.

In PostgreSQL, you could run the following query to view the date store on an Item:

select created from items limit 1;
>  2015-10-01 12:37:23.620442+00

It turns out, PostgreSQL has a handy date function that gives us just the date part of a datetime string.

select date(created) from items limit 1;
>  2015-10-01

Func() expressions (using in conjunction with F() expressions and the aggregate method will let us call postgres's date function from python!

from django.db.models import Func, F

# Note, this is PostgreSQL-specific.
# Build a queryset annotated with the date portion of the `created` datetime.
queryset = Item.objects.annotate(
  created_date=Func(F('created'), function='DATE')
)

# Now, we can query agains that annotation:
items = queryset.filter(created_date=date(2015, 10, 1))  # What we want!

Pretty Cool!

So there you have a number of ways to do date-based lookups in Django. These will probably get you where you want to go most of the time.

But wait! There's more! It turns out, you can build your own lookups. The Lookup API reference was introduced in Django 1.7, yet I've not dabbled with it. Stay Tuned, because in my next post, I'll see just how hard it is to implement that __date lookup (if nothing else gets in my way over the next few days).

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