How to create a triple-join table with Django

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?

From stackoverflow phloopy

• 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/)

  From Eli Courtwright

• 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.

  From Aaron Maenpaa

• 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)
  

  zgoda : This is quite new in Django (since 1.0).
  From zuber