django/tests/modeltests/expressions/models.py

"""
Tests for F() query expression syntax.
"""

from django.db import models

class Employee(models.Model):
    firstname = models.CharField(max_length=50)
    lastname = models.CharField(max_length=50)

    def __unicode__(self):
        return u'%s %s' % (self.firstname, self.lastname)

class Company(models.Model):
    name = models.CharField(max_length=100)
    num_employees = models.PositiveIntegerField()
    num_chairs = models.PositiveIntegerField()
    ceo = models.ForeignKey(
        Employee,
        related_name='company_ceo_set')
    point_of_contact = models.ForeignKey(
        Employee,
        related_name='company_point_of_contact_set',
        null=True)

    def __unicode__(self):
        return self.name


__test__ = {'API_TESTS': """
>>> from django.db.models import F

>>> Company(name='Example Inc.', num_employees=2300, num_chairs=5,
...     ceo=Employee.objects.create(firstname='Joe', lastname='Smith')).save()
>>> Company(name='Foobar Ltd.', num_employees=3, num_chairs=3,
...     ceo=Employee.objects.create(firstname='Frank', lastname='Meyer')).save()
>>> Company(name='Test GmbH', num_employees=32, num_chairs=1,
...     ceo=Employee.objects.create(firstname='Max', lastname='Mustermann')).save()

>>> company_query = Company.objects.values('name','num_employees','num_chairs').order_by('name','num_employees','num_chairs')

# We can filter for companies where the number of employees is greater than the
# number of chairs.
>>> company_query.filter(num_employees__gt=F('num_chairs'))
[{'num_chairs': 5, 'name': u'Example Inc.', 'num_employees': 2300}, {'num_chairs': 1, 'name': u'Test GmbH', 'num_employees': 32}]

# We can set one field to have the value of another field
# Make sure we have enough chairs
>>> _ = company_query.update(num_chairs=F('num_employees'))
>>> company_query
[{'num_chairs': 2300, 'name': u'Example Inc.', 'num_employees': 2300}, {'num_chairs': 3, 'name': u'Foobar Ltd.', 'num_employees': 3}, {'num_chairs': 32, 'name': u'Test GmbH', 'num_employees': 32}]

# We can perform arithmetic operations in expressions
# Make sure we have 2 spare chairs
>>> _ =company_query.update(num_chairs=F('num_employees')+2)
>>> company_query
[{'num_chairs': 2302, 'name': u'Example Inc.', 'num_employees': 2300}, {'num_chairs': 5, 'name': u'Foobar Ltd.', 'num_employees': 3}, {'num_chairs': 34, 'name': u'Test GmbH', 'num_employees': 32}]

# The relation of a foreign key can become copied over to an other foreign key.
>>> Company.objects.update(point_of_contact=F('ceo'))
3

>>> [c.point_of_contact for c in Company.objects.all()]
[<Employee: Joe Smith>, <Employee: Frank Meyer>, <Employee: Max Mustermann>]

>>> c = Company.objects.all()[0]
>>> c.point_of_contact = Employee.objects.create(firstname="Guido", lastname="van Rossum")
>>> c.save()

# F Expressions can also span joins
>>> Company.objects.filter(ceo__firstname=F('point_of_contact__firstname')).distinct()
[<Company: Foobar Ltd.>, <Company: Test GmbH>]

>>> _ = Company.objects.exclude(ceo__firstname=F('point_of_contact__firstname')).update(name='foo')
>>> Company.objects.exclude(ceo__firstname=F('point_of_contact__firstname')).get().name
u'foo'

>>> _ = Company.objects.exclude(ceo__firstname=F('point_of_contact__firstname')).update(name=F('point_of_contact__lastname'))
Traceback (most recent call last):
...
FieldError: Joined field references are not permitted in this query

# F expressions can be used to update attributes on single objects
>>> test_gmbh = Company.objects.get(name='Test GmbH')
>>> test_gmbh.num_employees
32
>>> test_gmbh.num_employees = F('num_employees') + 4
>>> test_gmbh.save()
>>> test_gmbh = Company.objects.get(pk=test_gmbh.pk)
>>> test_gmbh.num_employees
36

# F expressions cannot be used to update attributes which are foreign keys, or
# attributes which involve joins.
>>> test_gmbh.point_of_contact = None
>>> test_gmbh.save()
>>> test_gmbh.point_of_contact is None
True
>>> test_gmbh.point_of_contact = F('ceo')
Traceback (most recent call last):
...
ValueError: Cannot assign "<django.db.models.expressions.F object at ...>": "Company.point_of_contact" must be a "Employee" instance.

>>> test_gmbh.point_of_contact = test_gmbh.ceo
>>> test_gmbh.save()
>>> test_gmbh.name = F('ceo__last_name')
>>> test_gmbh.save()
Traceback (most recent call last):
...
FieldError: Joined field references are not permitted in this query

# F expressions cannot be used to update attributes on objects which do not yet
# exist in the database
>>> acme = Company(name='The Acme Widget Co.', num_employees=12, num_chairs=5,
...     ceo=test_gmbh.ceo)
>>> acme.num_employees = F('num_employees') + 16
>>> acme.save()
Traceback (most recent call last):
...
TypeError: int() argument must be a string or a number, not 'ExpressionNode'

"""}