Question

How can I make a ForeignKey refer back to the object itself? I’m trying :

Alias(MyBaseModel):
    type = models.ForeignKey('self')

a = Alias()
a.type = a
a.save()

But then when I run it :

(1048, "Column 'type_id' cannot be null")

I don’t want the type to be null, I want it to contain its own ID. I have tons of objects, but only 1 loops back to itself, so I really don’t want tot make it null. Ideas?

Answer 1

The problem is that when you did a.type = a, a did not yet exist in the database, therefore it has no pk.

One way around this is to save twice, first to save it into the database, referring to a dummy object that’s already in the database, then save it again once you can get it from the database. One problem with this is there’s sort of a chicken and egg problem of actually getting such an object into the database in the first place. I’d deal with this by means of creating a fixture to run each time you use sync-db, so that one such object exists and is valid.

Another option is to relax the not-null constraint and be diligent about getting them assigned.

Either way, inserting into such a table will always be a pain because you can’t get the PK to use without first inserting, and you can’t insert without having a key to use in the reference field, regardless of ORM or database or anything.

an alternative solution is to break the circular dependency and do something like:

AliasType(django.db.Model):
    pass

Alias(MyBaseModel):
    type = models.ForeignKey(AliasType)

a = Alias()
a_t = AliasType()
a_t.save()
a.type = a_t
a.save()

You can still get useful information by using django’s very smart accessors,

AliasType.objects.all()[0].alias_set

which can get you back to the original Alias object without having an explicit link to it.