Django's ManyToMany Relationship with Additional Fields

Question

I want to store some additional information in that, automatically created, ManyToMany join-table. How would I do that in Django?

In my case I have two tables: "Employees" and "Projects". What I want to store is how much each of the employees receives per hour of work in each of the projects, since those values are not the same. So, how would I do that?

What occurred to me was to, instead of the method "ManyToManyField", create explicitly a third class/table to store those new informations and to set its relationship with "Employees" and "Projects" using the "ForeignKey" method. I'm pretty sure it will work, but is this the best approach?

Solution

Here is example of what you want to achieve:

http://docs.djangoproject.com/en/dev/topics/db/models/#extra-fields-on-many-to-many-relationships

In case link ever breaks:

from django.db import models

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

    def __str__(self):              # __unicode__ on Python 2
        return self.name

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='Membership')

    def __str__(self):              # __unicode__ on Python 2
        return self.name

class Membership(models.Model):
    person = models.ForeignKey(Person)
    group = models.ForeignKey(Group)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)

Comments

[ravi404]

Possible duplicate of How to add column in ManyToMany Table (Django)

[TylerH]

@ravz Why shouldn't that one be closed as a dupe of this one, instead?

[ravi404]

@TylerH this is an older post. However both the questions have answers , i'd suggest merging these.

[Timo]

How can I access the date_joined in a template?Group.date_joined does not work.

[gruszczy]

Group date joined won't work, because Group itself doesn't join anything. Instead, you can get it's members and get their join time. for member in group.members.all():

[Dejell]

How can I use group.members.add() with the above approach?

[Devesh Khandelwal]

@Dejell the code samples in the link mentioned might help you.

[Sensei]

You can access the glue model object with <model>_set. So group.membership_set.all() for example. Note you always lowercase the entire model name to find this attribute and then add _set.

[Kabali]

through='Membership' , it is not creating the table in database. kindly help me.

[Tristan Brown]

Existing ManyToMany fields cannot be altered by adding or removing 'through=', according to the migration exception I just ran into.

[Tristan Brown]

To get around the "M2M fields cannot be altered" exception: stackoverflow.com/questions/26927705/…