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

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)

Answer 2

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 3

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 4

While trying to find a faster three-table join for my own Django models, I came across this question. By default, Django 1.1 uses INNER JOINs which can be slow on InnoDB. For a query like:

def event_users(event_name):
    return User.objects.filter(roles__events__name=event_name)

this might create the following SQL:

SELECT `user`.`id`, `user`.`name` FROM `user` INNER JOIN `roles` ON (`user`.`id` = `roles`.`user_id`) INNER JOIN `event` ON (`roles`.`event_id` = `event`.`id`) WHERE `event`.`name` = "event_name"

The INNER JOINs can be very slow compared with LEFT JOINs. An even faster query can be found under gimg1's answer: Mysql query to join three tables

SELECT `user`.`id`, `user`.`name` FROM `user`, `roles`, `event` WHERE `user`.`id` = `roles`.`user_id` AND `roles`.`event_id` = `event`.`id` AND `event`.`name` = "event_name"

However, you will need to use a custom SQL query: https://docs.djangoproject.com/en/dev/topics/db/sql/

In this case, it would look something like:

from django.db import connection
def event_users(event_name):
    cursor = connection.cursor()
    cursor.execute('select U.name from user U, roles R, event E' \
                   ' where U.id=R.user_id and R.event_id=E.id and E.name="%s"' % event_name)
    return [row[0] for row in cursor.fetchall()]