CODEWITHSUNDEEP
pythondjangodjango-modelsmany-to-many
Ron
Ron

Reputation: 23516

Django Many-to-Many (m2m) Relation to same model

I'd like to create a many-to-many relationship from and to a user class object.

I have something like this:

class MyUser(models.Model):
    ...
    blocked_users = models.ManyToManyField(MyUser, blank=True, null=True)

The question is if I can use the class reference inside itself. Or do I have to use "self" insead of "MyUser" in the ManyToManyField? Or is there another (and better) way to do it?

Upvotes: 86

Views: 48182

Answers (6)

DmitryBara
DmitryBara

Reputation: 501

Don't forget use symmetrical=False, if you use .clear() or .add() method for related objects and don't wanna object on other side of relation update own data in relation field.

some_field = models.ManyToManyField('self', symmetrical=False)

ref: Django Documentation: ManyToManyField.symmetrical

Upvotes: 31

sahama
sahama

Reputation: 679

I think it should be class name instead of self. because with using self like this

parent = models.ManyToManyField('self', null=True, blank=True)

when i add parent:

user1.parent.add(user2)

i have 2 record in database like this: enter image description here

and with using class name liken this:

parent = models.ManyToManyField('User', null=True, blank=True)

i have one record in database like this: enter image description here

note that i use uuid for pk and i use django 3.1

EDIT: as @shinra-tensei explained as comment in this answer we have to set symmetrical to False if we use self. documented in Django Documents: ManyToManyField.symmetrical

Upvotes: 6

Francisco Revilla
Francisco Revilla

Reputation: 31

If you use self or MyUser you will get a NameError in both cases. You should write "self" as string. See the example below:

class MyUser(models.Model):
    ...
    blocked_users = models.ManyToManyField("self", blank=True, null=True)

And do not forget to set the symmetrical attribute to False if the relationship is not symmetrical.

For further details check: https://docs.djangoproject.com/en/3.0/ref/models/fields/#django.db.models.ManyToManyField

Upvotes: 3

ruandao
ruandao

Reputation: 410

don't use 'self' in ManyToManyField, it will cause you object link each other, when use django form to submit it

class Tag(models.Model):
    ...
    subTag = models.ManyToManyField("self", blank=True)

 ...
 aTagForm.save()

and result:

 a.subTag == b
 b.subTag == a

Upvotes: 0

Goin
Goin

Reputation: 3964

class MyUser(models.Model):
    ...
    blocked_users = models.ManyToManyField("self", blank=True)

Upvotes: 61

Chris Pratt
Chris Pratt

Reputation: 239430

Technically, I'm pretty sure "MyUser" or "self" will work, as long as it's a string in either case. You just can't pass MyUser, the actual class.

However, the docs always use "self". Using "self" is not only more explicit about what's actually happening, but it's impervious to class name changes. For example, if you later changed MyUser to SomethingElse, you would then need to update any reference to "MyUser" as well. The problem is that since it's a string, your IDE will not alert you to the error, so there's a greater chance of your missing it. Using "self" will work no matter what the class' name is now or in the future.

Upvotes: 112

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