Django ORM经历多个多对多关系的方式

Dear people trying to help others,

亲爱的人们试图帮助他人，

I am trying to figure out how to get Django to do a join for me without writing custom SQL.

我试图弄清楚如何让Django为我做一个连接，而无需编写自定义SQL。

Let's say I have the following models

假设我有以下型号

class Parent(models.Model): 
  name =  models.CharField()
  children = models.ManyToManyField(Child, through="Parent_Child", related_name="parents")

class Parent_Child(models.Model):
  parent = models.ForeignKey(Parent, related_name='attached_children')
  child = models.ForeignKey(Child,  related_name='attached_parents')

class Child(models.Model): 
  name = models.CharField() 
  toys = models.ManyToManyField(Toy, hrough="Child_Toy", related_name="toy_owners")

class Child_Toy(models.Model): 
  child = models.ForeignKey(Child, related_name='attached_toys') 
  toy =  models.ForeignKey(Toy, related_name='toy_owner')

class Toy(models.Model): 
  name = models.CharField

A parent can have multiple children. A child can have multiple parents. A child can own multiple toys. Toys can be owned by multiple children.

父母可以有多个孩子。一个孩子可以有多个父母。孩子可以拥有多个玩具。玩具可以由多个孩子拥有。

I want to get a list of all toys owned by a Parent's Children.

我想获得父母子女拥有的所有玩具清单。

So, I can do things like: parent.children.all() and child.toys.all()

所以，我可以这样做：parent.children.all（）和child.toys.all（）

what I want to do is something like parent.children.toys.all() When I try to do this I get: AttributeError: 'ManyRelatedManager' object has no attribute 'toys'. I do understand the error - parent.children returns multiple records. This is expected. What I can't figure out is how to give Django the hint that I want it to add an additional join to its query.

我想做的是像parent.children.toys.all（）当我尝试这样做时，我得到：AttributeError：'ManyRelatedManager'对象没有属性'玩具'。我理解错误 - parent.children返回多条记录。这是预料之中的。我无法弄清楚的是如何给Django提示我希望它为其查询添加一个额外的连接。

Is there a way I can do this join within Django or do I need to go to custom SQL in order to do this?

有没有办法可以在Django中进行此连接，还是我需要转到自定义SQL才能执行此操作？

Please Note: The above is just meant to illustrate my issue, the actual models that I am using aren't that relevant. My issue is really trying to figure out how to join through multiple M2M relationships in Django without having to resort to SQL.

请注意：以上只是为了说明我的问题，我使用的实际模型并不相关。我的问题是试图弄清楚如何在Django中通过多个M2M关系加入，而不必诉诸SQL。

I appreciate your help in advance. Thanks!

我提前感谢您的帮助。谢谢！

2 个解决方案

#1

Simply write something like:

只需写下以下内容：

Toy.objects.filter(toy_owners__parents=parent)

#2

If you don't store extra information in the intermediate tables Parent_Child and Child_Toy you can just leave them out - Django will create them automatically for you. So a simplified setup would look like this:

如果你不在中间表Parent_Child和Child_Toy中存储额外的信息，你可以将它们遗漏--Django将自动为你创建它们。所以简化的设置看起来像这样：

class Parent(models.Model): 
    name =  models.CharField(max_length=80)
    children = models.ManyToManyField('Child', related_name="parent")

class Child(models.Model): 
    name = models.CharField(max_length=80) 
    toys = models.ManyToManyField('Toy', related_name="owner")

class Toy(models.Model): 
    name = models.CharField(max_length=80)

You can query the toys for a specific parent by using field lookups.

您可以使用字段查找查询特定父级的玩具。

Toy.objects.filter(owner__parent__id=1)

Or:

要么：

Toy.objects.filter(owner__parent=parent)

The resulting SQL looks something like this:

生成的SQL看起来像这样：

SELECT "toy"."id", "toy"."name" FROM "toy" 
    INNER JOIN "child_toys"
        ON ("toy"."id" = "child_toys"."toy_id") 
    INNER JOIN "child"
        ON ("child_toys"."child_id" = "child"."id") 
    INNER JOIN "parent_children" 
        ON ("child"."id" = "parent_children"."child_id")
    WHERE "parent_children"."parent_id" = 1

#1