Django中的模型如何获得具有反向关系的所有类的集合？

Question

考虑：

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) 

哪有一个，只给出了类食品，得到了一组具有“反向关系”的所有类的。即给定类食品，如何才能得到类猫,牛和人类。

我认为这是可能的，因为食品有三大 “反向关系”：Food.cat_set，Food.cow_set和Food.human_set。

帮助赞赏&谢谢！

Answer 1

要么

A）使用multiple table inheritance，并创建一个 “食” 的基类，即猫，牛和人继承。 B）使用Generic Relation，其中Food可以链接到任何其他模型。

这些都是有据可查的和官方支持的功能，您最好坚持让它们保持自己的代码清洁，避免变通方法并确保将来仍然支持它。

- EDIT（又名“如何成为一个声誉妓女”）

所以，这里是特定情况下的配方。

让我们假设你绝对需要猫，牛和人的单独模型。在现实世界的应用程序中，您想问自己为什么“类别”字段不能完成这项工作。

通过泛型关系可以更容易地到达“真实”类，所以这里是B的实现。我们不能在Person，Cat或Cow中拥有这个'food'字段，否则我们会遇到同样的问题。因此，我们将创建一个中间人“FoodConsumer”模型。如果我们不想为一个实例提供超过一种食物，我们将不得不编写额外的验证测试。现在

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)  

，以证明它让我们只写了这方面的工作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' 

""" 

Answer 2

在源代码中一些挖透露：

的Django/DB /模型/ options.py：

def get_all_related_objects(self, local_only=False): 

def get_all_related_many_to_many_objects(self, local_only=False) 

，并使用从上面的模型这些功能，你得假设：

>>> 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：我听说Model._meta是“不稳定”，或许不应该被在后的Django 1.0世界依靠。

感谢您的阅读。 :)