Why is unique good practise in 1:1 relationships?

Question

In the Grails Doc: http://grails.org/doc/2.3.x/guide/GORM.html It says that adding a unique constraint in a 1:1 is good practise.
So if a face has one nose, we should do:

class Face {
    static hasOne = [nose:Nose]
    static constraints = {
        nose unique: true
    }
}

But Why? Surely, the constraint is implicit in the cardinality?

So why should we do it?

Answer 1

The reason for putting the unique constraint is to ensure that two faces don't have the same nose. Since this is a 1:1 relationship the Id of the nose is kept on the face. That's why.

Answer 2

To be sure that only one nose knows the current face.

Without this constraint :

Face face = new Face();
Nose nose1 = new Nose();
face.nose = nose1;
face.save(flush: true);

Nose nose2 = new Nose();
face.nose = nose2;
face.save(flush: true); // no error, but in DB, nose1 and nose2 reference the same face_id