Django:模型通过中间模型有两个多对多关系 [英] Django: model has two ManyToMany relations through intermediate model

问题描述

所以我是Django新手，想描述一下场景:有一堆Persons，还有一堆Items，一个人通过Items 给另一个 Person.

So I am new to Django, and want to describe the scenario: there are a bunch of Persons, and there are a bunch of Items, and a person passes Items to another Person.

我有以下模型:

class Item(models.Model):
    title = models.CharField(max_length=1024, blank=False)

    def __unicode__(self):
        return self.title

class Person(models.Model):
    name = models.CharField(max_length=127, blank=False)
    out_item = models.ManyToManyField(
        Item,
        through='Event',
        through_fields=('from_user', 'item'),
        related_name='giver'
    )
    in_item = models.ManyToManyField(
        Item,
        through='Event',
        through_fields=('to_user', 'item'),
        related_name='receiver'
    )

    def __unicode__(self):
        return self.name

class Event(models.Model):
    item = models.ForeignKey(Item)
    from_user = models.ForeignKey(Person, related_name='event_as_giver')
    to_user = models.ForeignKey(Person, related_name='event_as_receiver')

但是 makemigrations 告诉我 app.Person: (models.E003) 该模型通过中间模型 'app.Event' 有两个多对多关系.

不知道我做错了什么?或者什么是实现场景的干净方法?也许我可以将 Event 分成 GiveEvent 和 ReceiveEvent ?但这在直觉上意义不大，因为当传递 item 时实际上只有一个事件.

I wonder what I did wrong? Or what is a clean way to achieve the scenario? Perhaps I can separate Event into GiveEvent and ReceiveEvent? But that just makes less sense intuitively, since there is actually only a single event when item is passed.

推荐答案

你所描述的听起来很合理.不允许这样做可能有技术原因；一个语义原因是每个ManyToManyField都意味着创建一个新表，并且不能有两个同名的表(即由同一个类表示).

What you're describing sounds reasonable enough. There may be a technical reason why that's disallowed; one semantic reason is that each ManyToManyField implies the creation of a new table, and there can't be two tables with the same name (i.e. represented by the same class).

另一种方法(更短更干)是这样的:

One alternative approach (shorter and more DRY) would be this:

class Person(models.Model): 
    name = models.CharField(max_length=127, blank=False)
    to_users = models.ManyToManyField(
        'self', 
        symmetrical=False, 
        related_name='from_users',
        through='Event', 
        through_fields=('from_user', 'to_user'),
    )

class Event(models.Model):
    item = models.ForeignKey(Item, related_name='events')
    from_user = models.ForeignKey(Person, related_name='events_as_giver')
    to_user = models.ForeignKey(Person, related_name='events_as_receiver')

表结构相同但描述符不同.访问相关的 people 稍微容易一些，但是访问相关的 items 就有点困难(例如，而不是 person.out_items.all() 你会说 Item.objects.filter(events__from_user=person).distinct()).

The table structure is the same but the descriptors are different. Accessing related people is a bit easier but accessing related items is a bit harder (for example, instead of person.out_items.all() you would say Item.objects.filter(events__from_user=person).distinct()).

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