如何获取Django模型的所有反向关系类集合？

Question

给定：

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)

如果只有一个叫做Food的类，怎么才能找到所有与它有“反向关系”的类呢？也就是说，给定Food这个类，怎么能找到Cat、Cow和Human这几个类。

我觉得这是可能的，因为Food有三个“反向关系”：Food.cat_set、Food.cow_set和Food.human_set。

谢谢大家的帮助！

Answer 1

经过一些对源代码的研究，我发现了：

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)

然后，假设你在上面的模型上使用这些函数，你可能会得到：

>>> 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，可能在 Django 1.0 之后不应该依赖它。

谢谢你的阅读。:)

Answer 2

要么

A) 使用多表继承，创建一个“Eater”基础类，让猫、牛和人都从这个类继承。

B) 使用通用关系，这样食物就可以和任何其他模型关联。

这两种方法都有详细的文档说明，并且是官方支持的功能，最好还是按照这些来做，这样可以保持代码的整洁，避免使用变通的方法，并且确保将来仍然会得到支持。

-- 编辑（也就是“如何提高声望”）

那么，这里有一个针对这种情况的解决方案。

假设你真的想为猫、牛和人创建独立的模型。在实际应用中，你应该问自己，为什么一个“类别”字段不能解决问题。

通过通用关系更容易找到“真实”的类，所以这里是B的实现。我们不能在“人”、“猫”或“牛”中直接添加“食物”字段，否则会遇到同样的问题。因此，我们将创建一个中介模型“FoodConsumer”。如果我们不想让一个实例有多于一种食物，我们还需要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，为了演示这一点，我们来写一个可工作的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'

"""