CODEWITHSUNDEEP
pythondjangorelationshipm2m
Nick Solarz
Nick Solarz

Reputation: 1

Django M2M Through extra fields with multiple models

I'm trying to figure out the best way to set up the following django model (genericised for security reasons).

ThingA:
    User(M2M through "UserRelation")

ThingB:
    User(M2M through "UserRelation")

ThingC:
    User(M2M through "UserRelation")

User:
    Login_name

UserRelation:
    User (foreginkey)
    Thing (foreignkey) #is this generic to any of the above "things"
    Privilege

I understand using "through" between two distinct models, but I'm not sure how to apply this to multiple models. Would I define a foreignkey for each of the "Thing" models in my UserRelation Model?

Upvotes: 0

Views: 541

Answers (1)

ouk
ouk

Reputation: 405

It looks like you are trying to setup a generic many-to-many relationship. There is a dedicated django app that you can be use for this purpose: django-gm2m

Here is how to use it in your generic case:

from django.db import models
from django.contrib.contenttypes.fields import GenericForeignKey

from gm2m import GM2MField


class ThingA(models.Model):
    pass

class ThingB(models.Model):
    pass

class ThingC(models.Model):
    pass

class User(models.Model):
     login_name = models.CharField(max_length=255)
     things = GM2MField(through='UserRelation')

class UserRelation(models.Model):
     user = models.ForeignKey(User)
     thing = GenericForeignKey(ct_field='thing_ct', fk_field='thing_fk')
     thing_ct = models.ForeignKey(ContentType)
     thing_fk = models.CharField(max_length=255)

     privilege = models.CharField(max_length=1)

You can now access all the things for a given user and all the User instances for a given 'thing', as well as the privilege attribute for each UserRelation instance.

This will additionally provide you with a handful of benefits (reverse relations, prefetching, etc.) you may need. A GM2MField basically behaves exactly like a django ManyToManyField.

Disclaimer: I am the author of django-gm2m

Upvotes: 2

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