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

Question

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?

what is the use defining many to many on same model?? anyone please — , Apr 10 '18 at 07:26
@ParaM For example if you have a tree-like structure. Master product has subproduct etc etc. — Ron, Apr 10 '18 at 11:14
*"what is the use defining many to many on same model??"* To allow each user to have their own list of other users they have blocked (or followed). — , Oct 24 '20 at 20:17

score 108 · Accepted Answer · answered Jul 30 '12 at 14:43

108

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.

answered Jul 30 '12 at 14:43

Chris Pratt

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what is the use defining many to many on same model?? anyone please – Apr 11 '18 at 04:51
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@ParaM An example: an Article model where you wish to specify "related articles" – BigglesZX Jun 05 '18 at 11:00
A personal example is an Agent with a field that specifies the agent that recruited them. – John R Perry Jun 09 '21 at 23:35

Goin · Answer 2 · 2016-05-24T16:29:06.050

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

edited May 24 '16 at 16:29

answered Jul 30 '12 at 12:06

Goin

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"self" doesn't work. Use blocked_users = models.ManyToManyField("MyUser", blank=True, null=True) – Kriss Jan 12 '14 at 21:12
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I didn't say "MyUser", I said "self": https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.ManyToManyField.symmetrical – Goin Jan 13 '14 at 07:19
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`null=True` has no effect on ManyToManyFields – asgaines Aug 16 '15 at 17:31

score 21 · Answer 3 · edited Mar 25 '23 at 02:09

21

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

edited Mar 25 '23 at 02:09

Snailedlt

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answered Mar 28 '21 at 09:10

DmitryBara

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This suggestion totally saved me! I was experiencing a cyclic dependency every time I would add a 'parent' relation to an entity and I was trying to debug it all day yesterday! – Dre May 10 '22 at 13:45

sahama · Answer 4 · 2020-10-24T07:14:42.413

6

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:

and with using class name liken this:

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

i have one record in database like this:

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

edited Oct 24 '20 at 07:14

answered Oct 21 '20 at 09:19

sahama

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What database adapter are you using? Might be a bug in there... – Ron Oct 22 '20 at 13:36
@Ron postgres and mysql both – sahama Oct 24 '20 at 06:58

score 3 · Answer 5 · answered May 11 '20 at 14:35

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

score 0 · Answer 6 · answered Jun 11 '19 at 04:09

0

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

answered Jun 11 '19 at 04:09

ruandao

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Any suggetions about it? I never had problems. – Ron Jun 11 '19 at 06:40
I was found it in my demo and finally i use ManyToManyField("Tag", blank=True) to fix it – ruandao Jun 12 '19 at 07:02
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If you don't want the objects to link each other, use the argument `symmetrical = False` when creating the field https://docs.djangoproject.com/en/2.2/ref/models/fields/#django.db.models.ManyToManyField.symmetrical , it exists to allow you to use the `'self'` name instead of the name of the model – Shinra tensei Sep 25 '19 at 08:52

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

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