Creating issue for discussion here at https://discourse.hibernate.org/t/hibernate-onetoone-with-mapsid-does-not-map-the-same-id-with-the-parent-entity/2093/9

Summary:
A child entity is mapped using @OneToOne with @MapsId to its parent entity.

For eg,

class Parent {
}
class Child {
    @OneToOne
    @MapsId
    private Parent parent;
}

The assumption is that child will refer to its correct parent in any case. However, following code will break this assumption.

Parent p = new Parent(); // new  parent
em.save(p);
em.flush();

Child c = em.find(Child.class, 1L); // child already have its original parent during its creation time
c.setParent(p); // updating to new parent
Child result = em.merge(c); 
result.getParent(); // new parent

This way it will continue to use new parent until the end of transaction.

@OneToOne with @MapsId should always resolves to correct association value.

Sample test cases which show the issue.
https://github.com/pmverma/hibernate-mapsid-test
https://github.com/vladmihalcea/high-performance-java-persistence/pull/36