Add composite foreign keys

[Csirmaz Bendegúz]

This is a follow up to #373 (CompositePrimaryKey).

Now that composite primary keys are merged, it would be great to be able to create foreign keys referencing them through the Django ORM.

My proposal is to add 2 parameters to ForeignKey: from_fields and to_fields.
They would map to the underlying ForeignObject's from_fields, to_fields parameters.

If a ForeignKey has multiple fields, it acts as a virtual field, meaning it doesn't create a database column automatically.

[Mariusz Felisiak]

Replying to Csirmaz Bendegúz:

My proposal is to add 2 parameters to ForeignKey: from_fields and to_fields.

Do we need these parameters? CompositePrimaryKey is always a primary key so its fields should be detected automatically, IMO, the following example should work:

class Release(models.Model):
    pk = models.CompositePrimaryKey("version", "name")
    version = models.IntegerField()
    name = models.CharField(max_length=20)


class RefRelease(models.Model):
    release = models.ForeignKey("Release", models.CASCADE)

[Csirmaz Bendegúz]

Replying to Mariusz Felisiak:

Do we need these parameters? CompositePrimaryKey is always a primary key so its fields should be detected automatically, IMO, the following example should work:

class Release(models.Model):
    pk = models.CompositePrimaryKey("version", "name")
    version = models.IntegerField()
    name = models.CharField(max_length=20)


class RefRelease(models.Model):
    release = models.ForeignKey("Release", models.CASCADE)

The issue with this example is RefRelease must have 2 fields matching the CompositePrimaryKey (an IntegerField() and a CharField(max_length=20)).

So release would need to create 2 fields implicitly and I think that's too restrictive.

We should allow sharing fields between composite foreign keys.

For example, in a multi-tenant app where each model has a CompositePrimaryKey("tenant_id", "id"), the tenant_id field should be shared between composite foreign keys.

Let me know what you think about this, I'm happy to consider alternatives.