To define a one-to-one relationship, use 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)
# 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:
>>> 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
<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 lookups across 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>]
Jul 20, 2015