django - ordering queryset by a calculated field

Question

I want to have a model with calculated fields that I can apply sorting on. For example, let's say that I have the following model:

class Foo(models.Model):
    A = models.IntegerField(..)
    B = models.IntegerField(..)
    C = models.ForeignKey(..)

I want to have a D and an E field that are calculated by the following formulas:

1. D = A - B
2. E = A - X (where X is a field of the relevant record of model C)

Implementing this would be trivial if I didn't need to apply sorting; I would just add properties to the model class. However, I need ordering by these fields.

A solution is to fetch all records into memory and do the sorting there, which I conceive a last resort (it will break things regarding pagination).

Is there a way to achieve what I'm trying? Any guidance is appreciated.

EDIT: Denormalization is a no-go. The value of field X changes very frequently and a lot of Foo records are related to one record of model C. An update of X will require thousands of updates of E.

Answer 1

If you would not mind some logic duplicaton, then the following will work:

Foo.objects.extra(select={'d_field': 'A - B'}).extra(order_by=['d_field'])

Answer 2

Please refrain from using extra() as it's meant to be deprecated in the future.

Since Django 1.7 you can use a combination of annotate() and order_by() to achieve this

Foo.objects.annotate(ordering=F('A') - F('B')).order_by('ordering')

There's also ungoing work to allow expressions to be used all over the ORM so the following should work in future versions of Django:

Foo.objects.order_by(F('A') - F('B'))

Answer 3

I would take a look at the extra method on Queryset and specify the order_by parameter.

Answer 4

As Simon says, you can now use expressions in queries, and those values will be calculated in the database. Here are the queries you asked about with the new sorting technique:

Foo.objects.order_by(F('a') - F('b'))
Foo.objects.order_by(F('a') - F('bar__x'))

Here's a complete runnable example that plays with these expressions:

# Tested with Django 1.9.2
import logging
import sys

import django
from django.apps import apps
from django.apps.config import AppConfig
from django.conf import settings
from django.db import connections, models, DEFAULT_DB_ALIAS
from django.db.models import F
from django.db.models.base import ModelBase
from django.db.models.functions import Concat, Value

from mock import patch, PropertyMock, MagicMock

NAME = 'udjango'


def main():

    setup()

    class Bar(models.Model):
        x = models.IntegerField()

    class Foo(models.Model):
        a = models.IntegerField()
        b = models.IntegerField()
        bar = models.ForeignKey(Bar)

    syncdb(Bar)
    syncdb(Foo)

    bar1 = Bar.objects.create(x=1)
    bar5 = Bar.objects.create(x=5)
    Foo.objects.create(a=10, b=3, bar=bar1)
    Foo.objects.create(a=13, b=3, bar=bar5)
    Foo.objects.create(a=15, b=9, bar=bar1)

    print(Foo.objects.annotate(ordering=F('a') - F('b'))
          .order_by('ordering').values_list('a', 'b', 'bar__x', 'ordering'))
    # >>> [(15, 9, 1, 6), (10, 3, 1, 7), (13, 3, 5, 10)]

    print(Foo.objects.annotate(ordering=F('a') - F('bar__x'))
          .order_by('ordering').values_list('a', 'b', 'bar__x', 'ordering'))
    # >>> [(13, 3, 5, 8), (10, 3, 1, 9), (15, 9, 1, 14)]

    print(Foo.objects.order_by(F('a') - F('b')).values_list('a', 'b', 'bar__x'))
    # >>> [(15, 9, 1), (10, 3, 1), (13, 3, 5)]

    print(Foo.objects.order_by(F('a') - F('bar__x')).values_list('a', 'b', 'bar__x'))
    # >>> [(13, 3, 5), (10, 3, 1), (15, 9, 1)]

    logging.info('Done.')


def setup():
    db_file = NAME + '.db'
    with open(db_file, 'w'):
        pass  # wipe the database
    settings.configure(
        DEBUG=True,
        DATABASES={
            DEFAULT_DB_ALIAS: {
                'ENGINE': 'django.db.backends.sqlite3',
                'NAME': db_file}},
        LOGGING={'version': 1,
                 'disable_existing_loggers': False,
                 'formatters': {
                    'debug': {
                        'format': '%(asctime)s[%(levelname)s]'
                                  '%(name)s.%(funcName)s(): %(message)s',
                        'datefmt': '%Y-%m-%d %H:%M:%S'}},
                 'handlers': {
                    'console': {
                        'level': 'DEBUG',
                        'class': 'logging.StreamHandler',
                        'formatter': 'debug'}},
                 'root': {
                    'handlers': ['console'],
                    'level': 'INFO'},
                 'loggers': {
                    "django.db": {"level": "DEBUG"}}})
    app_config = AppConfig(NAME, sys.modules['__main__'])
    apps.populate([app_config])
    django.setup()
    original_new_func = ModelBase.__new__

    # noinspection PyDecorator
    @staticmethod
    def patched_new(cls, name, bases, attrs):
        if 'Meta' not in attrs:
            class Meta:
                app_label = NAME
            attrs['Meta'] = Meta
        return original_new_func(cls, name, bases, attrs)
    ModelBase.__new__ = patched_new


def syncdb(model):
    """ Standard syncdb expects models to be in reliable locations.

    Based on https://github.com/django/django/blob/1.9.3
    /django/core/management/commands/migrate.py#L285
    """
    connection = connections[DEFAULT_DB_ALIAS]
    with connection.schema_editor() as editor:
        editor.create_model(model)

main()