######################## One-to-one relationships ######################## .. highlight:: pycon To define a one-to-one relationship, use :ref:`ref-onetoone`. In this example, a ``Place`` optionally can be a ``Restaurant``: .. code-block:: python from django.db import models class Place(models.Model): name = models.CharField(max_length=50) address = models.CharField(max_length=80) # On Python 3: def __str__(self): def __unicode__(self): return u"%s the place" % self.name class Restaurant(models.Model): place = models.OneToOneField(Place, primary_key=True) serves_hot_dogs = models.BooleanField() serves_pizza = models.BooleanField() # On Python 3: def __str__(self): def __unicode__(self): return u"%s the restaurant" % self.place.name class Waiter(models.Model): restaurant = models.ForeignKey(Restaurant) name = models.CharField(max_length=50) # On Python 3: def __str__(self): def __unicode__(self): return u"%s the waiter at %s" % (self.name, self.restaurant) What follows are examples of operations that can be performed using the Python API facilities. Create a couple of Places:: >>> p1 = Place(name='Demon Dogs', address='944 W. Fullerton') >>> p1.save() >>> p2 = Place(name='Ace Hardware', address='1013 N. Ashland') >>> p2.save() Create a Restaurant. Pass the ID of the "parent" object as this object's ID:: >>> r = Restaurant(place=p1, serves_hot_dogs=True, serves_pizza=False) >>> r.save() A Restaurant can access its place:: >>> r.place <Place: Demon Dogs the place> A Place can access its restaurant, if available:: >>> p1.restaurant <Restaurant: Demon Dogs the restaurant> p2 doesn't have an associated restaurant:: >>> p2.restaurant Traceback (most recent call last): ... DoesNotExist: Restaurant matching query does not exist. Set the place using assignment notation. Because place is the primary key on Restaurant, the save will create a new restaurant:: >>> r.place = p2 >>> r.save() >>> p2.restaurant <Restaurant: Ace Hardware the restaurant> >>> r.place <Place: Ace Hardware the place> Set the place back again, using assignment in the reverse direction:: >>> p1.restaurant = r >>> p1.restaurant <Restaurant: Demon Dogs the restaurant> Restaurant.objects.all() just returns the Restaurants, not the Places. Note that there are two restaurants - Ace Hardware the Restaurant was created in the call to r.place = p2:: >>> Restaurant.objects.all() [<Restaurant: Demon Dogs the restaurant>, <Restaurant: Ace Hardware the restaurant>] Place.objects.all() returns all Places, regardless of whether they have Restaurants:: >>> Place.objects.order_by('name') [<Place: Ace Hardware the place>, <Place: Demon Dogs the place>] You can query the models using :ref:`lookups across relationships <lookups-that-span-relationships>`:: >>> Restaurant.objects.get(place=p1) <Restaurant: Demon Dogs the restaurant> >>> Restaurant.objects.get(place__pk=1) <Restaurant: Demon Dogs the restaurant> >>> Restaurant.objects.filter(place__name__startswith="Demon") [<Restaurant: Demon Dogs the restaurant>] >>> Restaurant.objects.exclude(place__address__contains="Ashland") [<Restaurant: Demon Dogs the restaurant>] This of course works in reverse:: >>> Place.objects.get(pk=1) <Place: Demon Dogs the place> >>> Place.objects.get(restaurant__place__exact=p1) <Place: Demon Dogs the place> >>> Place.objects.get(restaurant=r) <Place: Demon Dogs the place> >>> Place.objects.get(restaurant__place__name__startswith="Demon") <Place: Demon Dogs the place> Add a Waiter to the Restaurant:: >>> w = r.waiter_set.create(name='Joe') >>> w.save() >>> w <Waiter: Joe the waiter at Demon Dogs the restaurant> Query the waiters:: >>> Waiter.objects.filter(restaurant__place=p1) [<Waiter: Joe the waiter at Demon Dogs the restaurant>] >>> Waiter.objects.filter(restaurant__place__name__startswith="Demon") [<Waiter: Joe the waiter at Demon Dogs the restaurant>]