Add parent_link to ForeignKey

[Victor Porton]

I propose to allow parent_link argument to ForeignKey.

This would mean that a model is derived from another model, but instead of OneToOneField we use ForeignKey what means that several instances of the derived model may share the same instance of the parent model.

Let

class A(models.Model):
    x = models.IntegerField()

class B(A):
    parent = ForeignKey(A, parent_link=True)
    y = models.IntegerField()

The rationale is the same as for existing model inheritance: I want to shorten code: Instead of b.parent.x write just b.x.

I see no other reason whatsoever (except that it a little complicates the understanding as every new feature does) to deny my feature. If it is done for OneToOneField why not to do the same for ForeignKey? There are no differences of arguments pro an contra for ForeignKey and for OneToOneField. Thus if this works for OneToOneField it should also be done for ForeignKey.

Also note that this new feature is useful for DB normalization: When the models structure changes, separating a parent object into a separate model, this feature would allow not to add .parent. every time x is used.

[Tim Graham]

Duplicate of #29691 (closed as wontfix). As described there, it would likely add much complexity.