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This patch does not remove all occurrences of the words in question. Rather, I went through all of the occurrences of the words listed below, and judged if they a) suggested the reader had some kind of knowledge/experience, and b) if they added anything of value (including tone of voice, etc). I left most of the words alone. I looked at the following words: - simply/simple - easy/easier/easiest - obvious - just - merely - straightforward - ridiculous Thanks to Carlton Gibson for guidance on how to approach this issue, and to Tim Bell for providing the idea. But the enormous lion's share of thanks go to Adam Johnson for his patient and helpful review.
152 lines
4.8 KiB
Plaintext
152 lines
4.8 KiB
Plaintext
========================
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One-to-one relationships
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========================
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To define a one-to-one relationship, use
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:class:`~django.db.models.OneToOneField`.
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In this example, a ``Place`` optionally can be a ``Restaurant``::
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from django.db import models
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class Place(models.Model):
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name = models.CharField(max_length=50)
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address = models.CharField(max_length=80)
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def __str__(self):
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return "%s the place" % self.name
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class Restaurant(models.Model):
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place = models.OneToOneField(
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Place,
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on_delete=models.CASCADE,
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primary_key=True,
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)
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serves_hot_dogs = models.BooleanField(default=False)
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serves_pizza = models.BooleanField(default=False)
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def __str__(self):
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return "%s the restaurant" % self.place.name
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class Waiter(models.Model):
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restaurant = models.ForeignKey(Restaurant, on_delete=models.CASCADE)
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name = models.CharField(max_length=50)
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def __str__(self):
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return "%s the waiter at %s" % (self.name, self.restaurant)
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What follows are examples of operations that can be performed using the Python
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API facilities.
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.. highlight:: pycon
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Create a couple of Places::
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>>> p1 = Place(name='Demon Dogs', address='944 W. Fullerton')
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>>> p1.save()
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>>> p2 = Place(name='Ace Hardware', address='1013 N. Ashland')
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>>> p2.save()
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Create a Restaurant. Pass the ID of the "parent" object as this object's ID::
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>>> r = Restaurant(place=p1, serves_hot_dogs=True, serves_pizza=False)
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>>> r.save()
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A Restaurant can access its place::
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>>> r.place
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<Place: Demon Dogs the place>
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A Place can access its restaurant, if available::
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>>> p1.restaurant
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<Restaurant: Demon Dogs the restaurant>
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p2 doesn't have an associated restaurant::
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>>> from django.core.exceptions import ObjectDoesNotExist
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>>> try:
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>>> p2.restaurant
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>>> except ObjectDoesNotExist:
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>>> print("There is no restaurant here.")
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There is no restaurant here.
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You can also use ``hasattr`` to avoid the need for exception catching::
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>>> hasattr(p2, 'restaurant')
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False
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Set the place using assignment notation. Because place is the primary key on
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Restaurant, the save will create a new restaurant::
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>>> r.place = p2
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>>> r.save()
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>>> p2.restaurant
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<Restaurant: Ace Hardware the restaurant>
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>>> r.place
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<Place: Ace Hardware the place>
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Set the place back again, using assignment in the reverse direction::
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>>> p1.restaurant = r
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>>> p1.restaurant
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<Restaurant: Demon Dogs the restaurant>
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Note that you must save an object before it can be assigned to a one-to-one
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relationship. For example, creating a ``Restaurant`` with unsaved ``Place``
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raises ``ValueError``::
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>>> p3 = Place(name='Demon Dogs', address='944 W. Fullerton')
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>>> Restaurant.objects.create(place=p3, serves_hot_dogs=True, serves_pizza=False)
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Traceback (most recent call last):
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...
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ValueError: save() prohibited to prevent data loss due to unsaved related object 'place'.
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Restaurant.objects.all() returns the Restaurants, not the Places. Note that
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there are two restaurants - Ace Hardware the Restaurant was created in the call
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to r.place = p2::
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>>> Restaurant.objects.all()
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<QuerySet [<Restaurant: Demon Dogs the restaurant>, <Restaurant: Ace Hardware the restaurant>]>
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Place.objects.all() returns all Places, regardless of whether they have
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Restaurants::
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>>> Place.objects.order_by('name')
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<QuerySet [<Place: Ace Hardware the place>, <Place: Demon Dogs the place>]>
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You can query the models using :ref:`lookups across relationships <lookups-that-span-relationships>`::
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>>> Restaurant.objects.get(place=p1)
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<Restaurant: Demon Dogs the restaurant>
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>>> Restaurant.objects.get(place__pk=1)
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<Restaurant: Demon Dogs the restaurant>
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>>> Restaurant.objects.filter(place__name__startswith="Demon")
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<QuerySet [<Restaurant: Demon Dogs the restaurant>]>
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>>> Restaurant.objects.exclude(place__address__contains="Ashland")
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<QuerySet [<Restaurant: Demon Dogs the restaurant>]>
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This of course works in reverse::
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>>> Place.objects.get(pk=1)
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<Place: Demon Dogs the place>
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>>> Place.objects.get(restaurant__place=p1)
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<Place: Demon Dogs the place>
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>>> Place.objects.get(restaurant=r)
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<Place: Demon Dogs the place>
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>>> Place.objects.get(restaurant__place__name__startswith="Demon")
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<Place: Demon Dogs the place>
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Add a Waiter to the Restaurant::
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>>> w = r.waiter_set.create(name='Joe')
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>>> w
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<Waiter: Joe the waiter at Demon Dogs the restaurant>
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Query the waiters::
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>>> Waiter.objects.filter(restaurant__place=p1)
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<QuerySet [<Waiter: Joe the waiter at Demon Dogs the restaurant>]>
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>>> Waiter.objects.filter(restaurant__place__name__startswith="Demon")
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<QuerySet [<Waiter: Joe the waiter at Demon Dogs the restaurant>]>
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