269 lines
8.8 KiB
Python
269 lines
8.8 KiB
Python
"""
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XX. Model inheritance
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Model inheritance exists in two varieties:
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- abstract base classes which are a way of specifying common
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information inherited by the subclasses. They don't exist as a separate
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model.
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- non-abstract base classes (the default), which are models in their own
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right with their own database tables and everything. Their subclasses
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have references back to them, created automatically.
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Both styles are demonstrated here.
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"""
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from django.db import models
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#
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# Abstract base classes
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#
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class CommonInfo(models.Model):
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name = models.CharField(max_length=50)
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age = models.PositiveIntegerField()
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class Meta:
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abstract = True
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ordering = ['name']
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def __unicode__(self):
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return u'%s %s' % (self.__class__.__name__, self.name)
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class Worker(CommonInfo):
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job = models.CharField(max_length=50)
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class Student(CommonInfo):
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school_class = models.CharField(max_length=10)
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class Meta:
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pass
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#
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# Multi-table inheritance
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#
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class Chef(models.Model):
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name = models.CharField(max_length=50)
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def __unicode__(self):
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return u"%s the chef" % self.name
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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 __unicode__(self):
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return u"%s the place" % self.name
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class Rating(models.Model):
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rating = models.IntegerField(null=True, blank=True)
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class Meta:
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abstract = True
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ordering = ['-rating']
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class Restaurant(Place, Rating):
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serves_hot_dogs = models.BooleanField()
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serves_pizza = models.BooleanField()
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chef = models.ForeignKey(Chef, null=True, blank=True)
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class Meta(Rating.Meta):
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db_table = 'my_restaurant'
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def __unicode__(self):
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return u"%s the restaurant" % self.name
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class ItalianRestaurant(Restaurant):
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serves_gnocchi = models.BooleanField()
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def __unicode__(self):
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return u"%s the italian restaurant" % self.name
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class Supplier(Place):
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customers = models.ManyToManyField(Restaurant, related_name='provider')
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def __unicode__(self):
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return u"%s the supplier" % self.name
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class ParkingLot(Place):
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# An explicit link to the parent (we can control the attribute name).
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parent = models.OneToOneField(Place, primary_key=True, parent_link=True)
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main_site = models.ForeignKey(Place, related_name='lot')
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def __unicode__(self):
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return u"%s the parking lot" % self.name
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__test__ = {'API_TESTS':"""
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# The Student and Worker models both have 'name' and 'age' fields on them and
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# inherit the __unicode__() method, just as with normal Python subclassing.
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# This is useful if you want to factor out common information for programming
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# purposes, but still completely independent separate models at the database
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# level.
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>>> w = Worker(name='Fred', age=35, job='Quarry worker')
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>>> w.save()
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>>> w2 = Worker(name='Barney', age=34, job='Quarry worker')
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>>> w2.save()
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>>> s = Student(name='Pebbles', age=5, school_class='1B')
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>>> s.save()
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>>> unicode(w)
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u'Worker Fred'
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>>> unicode(s)
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u'Student Pebbles'
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# The children inherit the Meta class of their parents (if they don't specify
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# their own).
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>>> Worker.objects.values('name')
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[{'name': u'Barney'}, {'name': u'Fred'}]
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# Since Student does not subclass CommonInfo's Meta, it has the effect of
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# completely overriding it. So ordering by name doesn't take place for Students.
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>>> Student._meta.ordering
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[]
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# However, the CommonInfo class cannot be used as a normal model (it doesn't
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# exist as a model).
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>>> CommonInfo.objects.all()
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Traceback (most recent call last):
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...
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AttributeError: type object 'CommonInfo' has no attribute 'objects'
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# The Place/Restaurant/ItalianRestaurant models, on the other hand, all exist
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# as independent models. However, the subclasses also have transparent access
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# to the fields of their ancestors.
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# Create a couple of Places.
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>>> p1 = Place(name='Master Shakes', address='666 W. Jersey')
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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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Test constructor for Restaurant.
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>>> r = Restaurant(name='Demon Dogs', address='944 W. Fullerton',serves_hot_dogs=True, serves_pizza=False, rating=2)
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>>> r.save()
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# Test the constructor for ItalianRestaurant.
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>>> c = Chef(name="Albert")
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>>> c.save()
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>>> ir = ItalianRestaurant(name='Ristorante Miron', address='1234 W. Ash', serves_hot_dogs=False, serves_pizza=False, serves_gnocchi=True, rating=4, chef=c)
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>>> ir.save()
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>>> ir.address = '1234 W. Elm'
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>>> ir.save()
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# Make sure Restaurant and ItalianRestaurant have the right fields in the right
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# order.
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>>> [f.name for f in Restaurant._meta.fields]
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['id', 'name', 'address', 'place_ptr', 'rating', 'serves_hot_dogs', 'serves_pizza', 'chef']
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>>> [f.name for f in ItalianRestaurant._meta.fields]
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['id', 'name', 'address', 'place_ptr', 'rating', 'serves_hot_dogs', 'serves_pizza', 'chef', 'restaurant_ptr', 'serves_gnocchi']
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>>> Restaurant._meta.ordering
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['-rating']
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# Even though p.supplier for a Place 'p' (a parent of a Supplier), a Restaurant
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# object cannot access that reverse relation, since it's not part of the
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# Place-Supplier Hierarchy.
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>>> Place.objects.filter(supplier__name='foo')
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[]
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>>> Restaurant.objects.filter(supplier__name='foo')
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Traceback (most recent call last):
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...
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FieldError: Cannot resolve keyword 'supplier' into field. Choices are: address, chef, id, italianrestaurant, lot, name, place_ptr, provider, rating, serves_hot_dogs, serves_pizza
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# Parent fields can be used directly in filters on the child model.
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>>> Restaurant.objects.filter(name='Demon Dogs')
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[<Restaurant: Demon Dogs the restaurant>]
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>>> ItalianRestaurant.objects.filter(address='1234 W. Elm')
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[<ItalianRestaurant: Ristorante Miron the italian restaurant>]
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# Filters against the parent model return objects of the parent's type.
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>>> Place.objects.filter(name='Demon Dogs')
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[<Place: Demon Dogs the place>]
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# Since the parent and child are linked by an automatically created
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# OneToOneField, you can get from the parent to the child by using the child's
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# name.
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>>> place = Place.objects.get(name='Demon Dogs')
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>>> place.restaurant
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<Restaurant: Demon Dogs the restaurant>
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>>> Place.objects.get(name='Ristorante Miron').restaurant.italianrestaurant
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<ItalianRestaurant: Ristorante Miron the italian restaurant>
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>>> Restaurant.objects.get(name='Ristorante Miron').italianrestaurant
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<ItalianRestaurant: Ristorante Miron the italian restaurant>
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# This won't work because the Demon Dogs restaurant is not an Italian
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# restaurant.
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>>> place.restaurant.italianrestaurant
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Traceback (most recent call last):
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...
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DoesNotExist: ItalianRestaurant matching query does not exist.
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# Related objects work just as they normally do.
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>>> s1 = Supplier(name="Joe's Chickens", address='123 Sesame St')
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>>> s1.save()
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>>> s1.customers = [r, ir]
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>>> s2 = Supplier(name="Luigi's Pasta", address='456 Sesame St')
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>>> s2.save()
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>>> s2.customers = [ir]
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# This won't work because the Place we select is not a Restaurant (it's a
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# Supplier).
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>>> p = Place.objects.get(name="Joe's Chickens")
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>>> p.restaurant
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Traceback (most recent call last):
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...
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DoesNotExist: Restaurant matching query does not exist.
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# But we can descend from p to the Supplier child, as expected.
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>>> p.supplier
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<Supplier: Joe's Chickens the supplier>
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>>> ir.provider.order_by('-name')
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[<Supplier: Luigi's Pasta the supplier>, <Supplier: Joe's Chickens the supplier>]
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>>> Restaurant.objects.filter(provider__name__contains="Chickens")
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[<Restaurant: Ristorante Miron the restaurant>, <Restaurant: Demon Dogs the restaurant>]
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>>> ItalianRestaurant.objects.filter(provider__name__contains="Chickens")
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[<ItalianRestaurant: Ristorante Miron the italian restaurant>]
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>>> park1 = ParkingLot(name='Main St', address='111 Main St', main_site=s1)
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>>> park1.save()
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>>> park2 = ParkingLot(name='Well Lit', address='124 Sesame St', main_site=ir)
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>>> park2.save()
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>>> Restaurant.objects.get(lot__name='Well Lit')
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<Restaurant: Ristorante Miron the restaurant>
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# The update() command can update fields in parent and child classes at once
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# (although it executed multiple SQL queries to do so).
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>>> Restaurant.objects.filter(serves_hot_dogs=True, name__contains='D').update(name='Demon Puppies', serves_hot_dogs=False)
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>>> r1 = Restaurant.objects.get(pk=r.pk)
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>>> r1.serves_hot_dogs == False
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True
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>>> r1.name
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u'Demon Puppies'
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# The values() command also works on fields from parent models.
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>>> d = {'rating': 4, 'name': u'Ristorante Miron'}
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>>> list(ItalianRestaurant.objects.values('name', 'rating')) == [d]
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True
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# select_related works with fields from the parent object as if they were a
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# normal part of the model.
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>>> from django import db
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>>> from django.conf import settings
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>>> settings.DEBUG = True
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>>> db.reset_queries()
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>>> ItalianRestaurant.objects.all()[0].chef
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<Chef: Albert the chef>
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>>> len(db.connection.queries)
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2
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>>> ItalianRestaurant.objects.select_related('chef')[0].chef
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<Chef: Albert the chef>
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>>> len(db.connection.queries)
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3
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>>> settings.DEBUG = False
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"""}
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