text: """
  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()
  
  # We can filter for companies where the number of employees is greater than the
  # number of chairs.
  
  >>> Company.objects.filter(num_employees__gt=F('num_chairs'))
  [<Company: Example Inc.>, <Company: Test GmbH>]
  
  # 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
  
  """}
  