11👍
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)
3👍
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/)
0👍
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.
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0👍
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()]
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