4

鉴于：

from django.db import models

class Food(models.Model):
     """Food, by name."""
     name = models.CharField(max_length=25)

class Cat(models.Model):
     """A cat eats one type of food"""
     food = models.ForeignKey(Food)

class Cow(models.Model):
     """A cow eats one type of food"""
     food = models.ForeignKey(Food)

class Human(models.Model):
     """A human may eat lots of types of food"""
     food = models.ManyToManyField(Food)

仅给定 Food 类，如何获得与它具有“反向关系”的所有类的集合。即给定类Food，如何获得Cat、Cow和Human类。

我认为这是可能的，因为 Food 具有三个“反向关系”：Food.cat_set、Food.cow_set和Food.human_set。

感谢您的帮助，谢谢！

4

2 回答 2

14

对源代码的一些挖掘揭示了：

django/db/models/options.py：

def get_all_related_objects(self, local_only=False):

def get_all_related_many_to_many_objects(self, local_only=False)

并且，在上面的模型上使用这些函数，您假设得到：

>>> Food._meta.get_all_related_objects()
[<RelatedObject: app_label:cow related to food>,
    <RelatedObject: app_label:cat related to food>,]

>>> Food._meta.get_all_related_many_to_many_objects()
[<RelatedObject: app_label:human related to food>,]

# and, per django/db/models/related.py
# you can retrieve the model with
>>> Food._meta.get_all_related_objects()[0].model
<class 'app_label.models.Cow'>

注意：我听说 Model._meta 是“不稳定的”，在 Django-1.0 之后的世界中可能不应该依赖它。

谢谢阅读。:)

于 2008-11-11T02:00:20.570 回答
7

任何一个

A) 使用多表继承并创建一个 Cat、Cow 和 Human 继承自的“Eater”基类。

B）使用通用关系，其中食物可以链接到任何其他模型。

这些都是有据可查且官方支持的功能，您最好坚持使用它们以保持自己的代码干净，避免变通方法，并确保将来仍会支持它。

-- 编辑（又名“如何成为名声妓女”）

因此，这是针对该特定情况的秘诀。

假设您绝对想要 Cat、Cow 和 Human 的单独模型。在现实世界的应用程序中，您想问自己为什么“类别”字段不能胜任这项工作。

通过泛型关系更容易到达“真实”类，所以这里是 B 的实现。我们不能在 Person、Cat 或 Cow 中拥有那个“食物”字段，否则我们会遇到同样的问题。所以我们将创建一个中介“FoodConsumer”模型。如果我们不想要一个实例以上的食物，我们将不得不编写额外的验证测试。

from django.db import models
from django.contrib.contenttypes.models import ContentType
from django.contrib.contenttypes import generic

class Food(models.Model):
     """Food, by name."""
     name = models.CharField(max_length=25)

# ConsumedFood has a foreign key to Food, and a "eaten_by" generic relation
class ConsumedFood(models.Model):
    food = models.ForeignKey(Food, related_name="eaters")
    content_type = models.ForeignKey(ContentType, null=True)
    object_id = models.PositiveIntegerField(null=True)
    eaten_by = generic.GenericForeignKey('content_type', 'object_id')

class Person(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    birth_date = models.DateField()
    address = models.CharField(max_length=100)
    city = models.CharField(max_length=50)
    foods = generic.GenericRelation(ConsumedFood)

class Cat(models.Model):
    name = models.CharField(max_length=50)
    foods = generic.GenericRelation(ConsumedFood)    

class Cow(models.Model):
    farmer = models.ForeignKey(Person)
    foods = generic.GenericRelation(ConsumedFood)

现在，为了演示它，让我们编写这个工作doctest：

"""
>>> from models import *

Create some food records

>>> weed = Food(name="weed")
>>> weed.save()

>>> burger = Food(name="burger")
>>> burger.save()

>>> pet_food = Food(name="Pet food")
>>> pet_food.save()

John the farmer likes burgers

>>> john = Person(first_name="John", last_name="Farmer", birth_date="1960-10-12")
>>> john.save()
>>> john.foods.create(food=burger)
<ConsumedFood: ConsumedFood object>

Wilma the cow eats weed

>>> wilma = Cow(farmer=john)
>>> wilma.save()
>>> wilma.foods.create(food=weed)
<ConsumedFood: ConsumedFood object>

Felix the cat likes pet food

>>> felix = Cat(name="felix")
>>> felix.save()
>>> pet_food.eaters.create(eaten_by=felix)
<ConsumedFood: ConsumedFood object>

What food john likes again ?
>>> john.foods.all()[0].food.name
u'burger'

Who's getting pet food ?
>>> living_thing = pet_food.eaters.all()[0].eaten_by
>>> isinstance(living_thing,Cow)
False
>>> isinstance(living_thing,Cat)
True

John's farm is in fire ! He looses his cow.
>>> wilma.delete()

John is a lot poorer right now
>>> john.foods.clear()
>>> john.foods.create(food=pet_food)
<ConsumedFood: ConsumedFood object>

Who's eating pet food now ?
>>> for consumed_food in pet_food.eaters.all():
...    consumed_food.eaten_by
<Cat: Cat object>
<Person: Person object>

Get the second pet food eater
>>> living_thing = pet_food.eaters.all()[1].eaten_by

Try to find if it's a person and reveal his name
>>> if isinstance(living_thing,Person): living_thing.first_name
u'John'

"""
于 2008-11-11T03:48:52.053 回答