======================== One-to-one relationships ======================== To define a one-to-one relationship, use :class:`~django.db.models.OneToOneField`. In this example, a ``Place`` optionally can be a ``Restaurant``:: from django.db import models class Place(models.Model): name = models.CharField(max_length=50) address = models.CharField(max_length=80) def __str__(self): # __unicode__ on Python 2 return "%s the place" % self.name class Restaurant(models.Model): place = models.OneToOneField( Place, on_delete=models.CASCADE, primary_key=True, ) serves_hot_dogs = models.BooleanField(default=False) serves_pizza = models.BooleanField(default=False) def __str__(self): # __unicode__ on Python 2 return "%s the restaurant" % self.place.name class Waiter(models.Model): restaurant = models.ForeignKey(Restaurant, on_delete=models.CASCADE) name = models.CharField(max_length=50) def __str__(self): # __unicode__ on Python 2 return "%s the waiter at %s" % (self.name, self.restaurant) What follows are examples of operations that can be performed using the Python API facilities. .. highlight:: pycon 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 A Place can access its restaurant, if available:: >>> p1.restaurant p2 doesn't have an associated restaurant:: >>> from django.core.exceptions import ObjectDoesNotExist >>> try: >>> p2.restaurant >>> except ObjectDoesNotExist: >>> print("There is no restaurant here.") There is no restaurant here. You can also use ``hasattr`` to avoid the need for exception catching:: >>> hasattr(p2, 'restaurant') False 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 >>> r.place Set the place back again, using assignment in the reverse direction:: >>> p1.restaurant = r >>> p1.restaurant Note that you must save an object before it can be assigned to a one-to-one relationship. For example, creating a ``Restaurant`` with unsaved ``Place`` raises ``ValueError``:: >>> p3 = Place(name='Demon Dogs', address='944 W. Fullerton') >>> Restaurant.objects.create(place=p3, serves_hot_dogs=True, serves_pizza=False) Traceback (most recent call last): ... ValueError: save() prohibited to prevent data loss due to unsaved related object 'place'. 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() , ]> Place.objects.all() returns all Places, regardless of whether they have Restaurants:: >>> Place.objects.order_by('name') , ]> You can query the models using :ref:`lookups across relationships `:: >>> Restaurant.objects.get(place=p1) >>> Restaurant.objects.get(place__pk=1) >>> Restaurant.objects.filter(place__name__startswith="Demon") ]> >>> Restaurant.objects.exclude(place__address__contains="Ashland") ]> This of course works in reverse:: >>> Place.objects.get(pk=1) >>> Place.objects.get(restaurant__place=p1) >>> Place.objects.get(restaurant=r) >>> Place.objects.get(restaurant__place__name__startswith="Demon") Add a Waiter to the Restaurant:: >>> w = r.waiter_set.create(name='Joe') >>> w Query the waiters:: >>> Waiter.objects.filter(restaurant__place=p1) ]> >>> Waiter.objects.filter(restaurant__place__name__startswith="Demon") ]>