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Conditional Expressions

Conditional expressions let you use if … elif … else logic within filters, annotations, aggregations, and updates. A conditional expression evaluates a series of conditions for each row of a table and returns the matching result expression. Conditional expressions can also be combined and nested like other expressions.

The conditional expression classes

We’ll be using the following model in the subsequent examples:

Code
from django.db import models

class Client(models.Model):
    REGULAR = 'R'
    GOLD = 'G'
    PLATINUM = 'P'
    ACCOUNT_TYPE_CHOICES = (
        (REGULAR, 'Regular'),
        (GOLD, 'Gold'),
        (PLATINUM, 'Platinum'),
    )
    name = models.CharField(max_length=50)
    registered_on = models.DateField()
    account_type = models.CharField(
        max_length=1,
        choices=ACCOUNT_TYPE_CHOICES,
        default=REGULAR,
    )

When

class When(condition=None, then=None, **lookups)

A When() object is used to encapsulate a condition and its result for use in the conditional expression. Using a When() object is similar to using the filter() method. The condition can be specified using field lookups or Q objects. The result is provided using the then keyword.

Some examples:

Code
>>> from django.db.models import When, F, Q
>>> # String arguments refer to fields; the following two examples are equivalent:
>>> When(account_type=Client.GOLD, then='name')
>>> When(account_type=Client.GOLD, then=F('name'))
>>> # You can use field lookups in the condition
>>> from datetime import date
>>> When(registered_on__gt=date(2014, 1, 1),
...      registered_on__lt=date(2015, 1, 1),
...      then='account_type')
>>> # Complex conditions can be created using Q objects
>>> When(Q(name__startswith="John") | Q(name__startswith="Paul"),
...      then='name')

Keep in mind that each of these values can be an expression.

Case

class Case(*cases, **extra)

A Case() expression is like the if … elif … else statement in Python. Each condition in the provided When() objects is evaluated in order, until one evaluates to a truthful value. The result expression from the matching When() object is returned.

A simple example:

Code
>>>
>>> from datetime import date, timedelta
>>> from django.db.models import CharField, Case, Value, When
>>> Client.objects.create(
...     name='Jane Doe',
...     account_type=Client.REGULAR,
...     registered_on=date.today() - timedelta(days=36))
>>> Client.objects.create(
...     name='James Smith',
...     account_type=Client.GOLD,
...     registered_on=date.today() - timedelta(days=5))
>>> Client.objects.create(
...     name='Jack Black',
...     account_type=Client.PLATINUM,
...     registered_on=date.today() - timedelta(days=10 * 365))
>>> # Get the discount for each Client based on the account type
>>> Client.objects.annotate(
...     discount=Case(
...         When(account_type=Client.GOLD, then=Value('5%')),
...         When(account_type=Client.PLATINUM, then=Value('10%')),
...         default=Value('0%'),
...         output_field=CharField(),
...     ),
... ).values_list('name', 'discount')
[('Jane Doe', '0%'), ('James Smith', '5%'), ('Jack Black', '10%')]

Case() accepts any number of When() objects as individual arguments. Other options are provided using keyword arguments. If none of the conditions evaluate to TRUE, then the expression given with the default keyword argument is returned. If a default argument isn’t provided, None is used.

If we wanted to change our previous query to get the discount based on how long the Client has been with us, we could do so using lookups:

Code
>>> a_month_ago = date.today() - timedelta(days=30)
>>> a_year_ago = date.today() - timedelta(days=365)
>>> # Get the discount for each Client based on the registration date
>>> Client.objects.annotate(
...     discount=Case(
...         When(registered_on__lte=a_year_ago, then=Value('10%')),
...         When(registered_on__lte=a_month_ago, then=Value('5%')),
...         default=Value('0%'),
...         output_field=CharField(),
...     )
... ).values_list('name', 'discount')
[('Jane Doe', '5%'), ('James Smith', '0%'), ('Jack Black', '10%')]

Advanced queries

Conditional expressions can be used in annotations, aggregations, lookups, and updates. They can also be combined and nested with other expressions. This allows you to make powerful conditional queries.

Conditional update

Let’s say we want to change the account_type for our clients to match their registration dates. We can do this using a conditional expression and the update() method:

Code
>>> a_month_ago = date.today() - timedelta(days=30)
>>> a_year_ago = date.today() - timedelta(days=365)
>>> # Update the account_type for each Client from the registration date
>>> Client.objects.update(
...     account_type=Case(
...         When(registered_on__lte=a_year_ago,
...              then=Value(Client.PLATINUM)),
...         When(registered_on__lte=a_month_ago,
...              then=Value(Client.GOLD)),
...         default=Value(Client.REGULAR)
...     ),
... )
>>> Client.objects.values_list('name', 'account_type')
[('Jane Doe', 'G'), ('James Smith', 'R'), ('Jack Black', 'P')]

Conditional aggregation

What if we want to find out how many clients there are for each account_type? We can nest conditional expression within aggregate functions to achieve this:

Code
>>> # Create some more Clients first so we can have something to count
>>> Client.objects.create(
...     name='Jean Grey',
...     account_type=Client.REGULAR,
...     registered_on=date.today())
>>> Client.objects.create(
...     name='James Bond',
...     account_type=Client.PLATINUM,
...     registered_on=date.today())
>>> Client.objects.create(
...     name='Jane Porter',
...     account_type=Client.PLATINUM,
...     registered_on=date.today())
>>> # Get counts for each value of account_type
>>> from django.db.models import IntegerField, Sum
>>> Client.objects.aggregate(
...     regular=Sum(
...         Case(When(account_type=Client.REGULAR, then=1),
...              output_field=IntegerField())
...     ),
...     gold=Sum(
...         Case(When(account_type=Client.GOLD, then=1),
...              output_field=IntegerField())
...     ),
...     platinum=Sum(
...         Case(When(account_type=Client.PLATINUM, then=1),
...              output_field=IntegerField())
...     )
... )
{'regular': 2, 'gold': 1, 'platinum': 3}
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