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.
• Us

Answer 1

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()]