How to use Django database function expressions directly

Django has a many built-in database functions and a documented Func API for writing your own.

Whilst writing a custom Func subclass may sometimes be necessary, I learnt that there’s many cases when you can instantiate Func with the necessary arguments to get what you need. For example, take the following model.

from django.contrib.postgres.fields import ArrayField
from django.db import models

class Post(models.Model):
    tags = ArrayField(models.CharField(max_length=200))

Making use of the array_shuffle Postgres Array Function and an annotation, each of the post’s tags are in a randomised order.

from django.db.models import F, Func

posts = Post.objects.annotate(
    tags_shuffle=Func(F("tags"), function="ARRAY_SHUFFLE"),
)

A second, slightly more contrived example.

from django.db import models

class Post(models.Model):
    published_year = models.IntegerField()
    published_month = models.IntegerField()
    published_day = models.IntegerField()

This time using the make_date Postgres’ Date/Time Function to get a date object from the post’s published values.

from django.db.models import DateField, F, Func

posts = Post.objects.annotate(
    published_date=Func(
        F("published_year"),
        F("published_month"),
        F("published_day"),
        output_field=DateField(),
        function="MAKE_DATE",
    )
)