How to create a triple-join table with Django

Question

Using Django's built in models, how would one create a triple-join between three models.

For example:

• Users, Roles, and Events are the models.
• Users have many Roles, and Roles many Users. (ManyToMany)
• Events have many Users, and Users many Events. (ManyToMany)
• But for any given Event, any User may have only one Role.

How can this be represented in the model?

Answer 1

I'd recommend just creating an entirely separate model for this.

class Assignment(Model):
    user = ForeignKey(User)
    role = ForeignKey(Role)
    event = ForeignKey(Event)

This lets you do all the usual model stuff, such as

user.assignment_set.filter(role__name="Chaperon")
role.assignment_set.filter(event__name="Silly Walkathon")

The only thing left is to enforce your one-role-per-user-per-event restriction. You can do this in the Assignment class by either overriding the save method (http://docs.djangoproject.com/en/dev/topics/db/models/#overriding-predefined-model-methods) or using signals (http://docs.djangoproject.com/en/dev/topics/signals/)

Answer 2

I'd model Role as an association class between Users and Roles, thus,

class User(models.Model):
     ...

class Event(models.Model):
     ...

class Role(models.Model):
     user = models.ForeignKey(User)
     event = models.ForeignKey(Event)

And enforce the one role per user per event in either a manager or SQL constraints.

Answer 3

zacherates writes:

I'd model Role as an association class between Users and Roles (...)

I'd also reccomed this solution, but you can also make use of some syntactical sugar provided by Django: ManyToMany relation with extra fields.

Example:

class User(models.Model):
    name = models.CharField(max_length=128)

class Event(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(User, through='Role')

    def __unicode__(self):
        return self.name

class Role(models.Model):
    person = models.ForeignKey(User)
    group = models.ForeignKey(Event)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)