使用“直通"与django-gm2m反向关系.关系 [英] Reverse relations with django-gm2m using "through" relation

问题描述

我不知道如何使用django-gm2m反向遵循多对多关系.这是models.py的示例:

I don't understand how to follow many-to-many relations in the reverse direction with django-gm2m. Here is an example of an models.py:

from django.db import models
from django.contrib.contenttypes.models import ContentType
from django.contrib.contenttypes.fields import GenericForeignKey
from gm2m import GM2MField

class A(models.Model):
    pass

class B(models.Model):
    pass

class X(models.Model):
    things = GM2MField()

class Y(models.Model):
    things = GM2MField(through='Yrel')

class Yrel(models.Model):
    y = models.ForeignKey(Y)
    thing = GenericForeignKey(ct_field='thing_ct', fk_field='thing_fk')
    thing_ct = models.ForeignKey(ContentType)
    thing_fk = models.CharField(max_length=255)
    timestamp = models.DateTimeField(auto_now_add=True)

X和Y都具有事物"，其中包含多个任意对象.是我遇到问题的Y，而X仅用于比较.

X and Y both have "things" which contains several arbitrary objects. It is Y I have problems with, and X is only for comparison.

我有一些要测试的对象.

I have a few objects to test with.

a1, a2, b1 = A(), A(), B()
a1.save()
a2.save()
b1.save()

等有了X班，我就可以做到

etc. With the class X I can do

x1, x2 = X(), X()
x1.save()
x2.save()
x1.things.add(a1, b1)
x2.things.add(a1)

，然后用x1.things.all()等返回添加的内容.要反向执行，请像a1.x_set.count()一样使用x_set.

and then get the added things back with x1.things.all() etc. To go in the reverse direction I use x_set as in a1.x_set.count().

到目前为止，一切都很好.对于使用通过"的"Y"，我会这样做

So far so good. With "Y" that uses "through" I do

y1 = Y()
y1.save()
Yrel(y=y1, thing=a1).save()
Yrel(y=y1, thing=a2).save()

添加两个事物"，然后我可以再次使用y1.things.all()来获取列表.但是，如何从a1进行反向查找以查看其使用位置?

to add two "things", and then I can get the list back with y1.things.all() again. But how can I do a reverse lookup from a1 to see where it is used?

推荐答案

如 django-gm2m只能在将实例添加到*_set后(与对X对象所做的一样)创建相关的反向关系，因为它不能知道在哪个模型上需要反向关系.

As described in the documentation django-gm2m can only create the related reverse relation after you have added an instance to the *_set (as you did with the X objects), as it can't know on which models the reverse relation is needed.

如果您想在没有添加任何内容之前访问反向关系，则需要指定应在哪些模型上创建:

If you would like to access the reverse relations without having added something before you need to specify on which models they should get created:

class Y(models.Model):
    things = GM2MField(through='Yrel', A, B)

这在某种程度上类似于Django的行为，在该行为中，您还必须为GenericForeignKey

This somehow resembles Django's behaviour where you would also have to create the reverse relation for GenericForeignKey manually.

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