One-to-one relationships

    In this example, a optionally can be a Restaurant:

    What follows are examples of operations that can be performed using the Python API facilities.

    Create a couple of Places:

    1. >>> p1 = Place(name='Demon Dogs', address='944 W. Fullerton')
    2. >>> p1.save()
    3. >>> p2 = Place(name='Ace Hardware', address='1013 N. Ashland')
    4. >>> p2.save()

    Create a Restaurant. Pass the “parent” object as this object’s primary key:

    1. >>> r = Restaurant(place=p1, serves_hot_dogs=True, serves_pizza=False)
    2. >>> r.save()

    A Restaurant can access its place:

    1. >>> r.place
    2. <Place: Demon Dogs the place>
    1. >>> p1.restaurant
    2. <Restaurant: Demon Dogs the restaurant>

    p2 doesn’t have an associated restaurant:

    You can also use hasattr to avoid the need for exception catching:

    1. >>> hasattr(p2, 'restaurant')
    2. False

    Set the place using assignment notation. Because place is the primary key on Restaurant, the save will create a new restaurant:

    1. >>> r.place = p2
    2. >>> r.save()
    4. >>> r.place
    5. <Place: Ace Hardware the place>

    Set the place back again, using assignment in the reverse direction:

    1. >>> p1.restaurant = r
    2. >>> p1.restaurant
    3. <Restaurant: Demon Dogs the 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:

    1. >>> p3 = Place(name='Demon Dogs', address='944 W. Fullerton')
    2. >>> Restaurant.objects.create(place=p3, serves_hot_dogs=True, serves_pizza=False)
    3. Traceback (most recent call last):
    4. ...
    5. ValueError: save() prohibited to prevent data loss due to unsaved related object 'place'.

    Place.objects.all() returns all Places, regardless of whether they have Restaurants:

    1. >>> Place.objects.order_by('name')
    2. <QuerySet [<Place: Ace Hardware the place>, <Place: Demon Dogs the place>]>

    You can query the models using lookups across relationships:

    2. <Restaurant: Demon Dogs the restaurant>
    3. >>> Restaurant.objects.get(place__pk=1)
    4. >>> Restaurant.objects.filter(place__name__startswith="Demon")
    5. <QuerySet [<Restaurant: Demon Dogs the restaurant>]>
    6. >>> Restaurant.objects.exclude(place__address__contains="Ashland")
    7. <QuerySet [<Restaurant: Demon Dogs the restaurant>]>

    This also works in reverse:

    1. >>> Place.objects.get(pk=1)
    2. <Place: Demon Dogs the place>
    3. >>> Place.objects.get(restaurant__place=p1)
    4. <Place: Demon Dogs the place>
    5. >>> Place.objects.get(restaurant=r)
    6. <Place: Demon Dogs the place>
    7. >>> Place.objects.get(restaurant__place__name__startswith="Demon")
    8. <Place: Demon Dogs the place>

    Add a Waiter to the Restaurant:

    1. >>> w = r.waiter_set.create(name='Joe')
    3. <Waiter: Joe the waiter at Demon Dogs the restaurant>

    Query the waiters: