3 table many to many relationship

[Malik Rumi]

I was designing the models I will need for this project. I designed an intermediate table for two models, A and B, and then started to sketch out an intermediate table for two other models, A and C, when I realized that these two intermediate tables both use A, and further, the information in the second intermediate table will be a lot more valuable it if also shows the relationship C has to B.

I looked at the Many to Many documentation on the official Django site, but I don’t see a discussion of this three table option. I have seen it elsewhere, so I assume it can be done. What I don’t assume is the impact this has on performance and other issues I might not even anticipate. So, my questions:

   Can this three sided many to many intermediate table be created in Django?

    If it can, is it advisable, or are there better / more efficient ways of doing this, like with two intermediate tables as I was originally thinking?

thx.

[Simon Charette]

Django allows you to explicitly specify an intermediary model for many-to-many relationships with the `through` option.

class A(models.Model):
    b_set = models.ManyToMany('B', related_name='a_set', through='R')
    c_set = models.ManyToMany('C', related_name='a_set', through='R')

class B(models.Model):
    pass

class C(models.Model):
    pass

class R(models.Model):
    a = models.ForeignKey('A')
    b = models.ForeignKey('B')
    c = models.ForeignKey('C')

However you'll loose the ability of directly adding objects to relationships (A().b_set.create() won't work).
You'll need to explicitly create `R` instances instead: R.objects.create(a=a, b=b, c=c).

Simon

[Malik Rumi]

Merci beaucoup

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