PLs add into documentation
5.1.2.1. Composite identifier

You can define a composite primary key through several syntaxes:

use a component type to represent the identifier and map it as a property in the entity: you then annotated the property as @EmbeddedId. The component type has to be Serializable.
map multiple properties as @Id properties: the identifier type is then the entity class itself and needs to be Serializable. This approach is unfortunately not standard and only supported by Hibernate.
map multiple properties as @Id properties and declare an external class to be the identifier type. This class, which needs to be Serializable, is declared on the entity via the @IdClass annotation. The identifier type must contain the same properties as the identifier properties of the entity: each property name must be the same, its type must be the same as well if the entity property is of a basic type, its type must be the type of the primary key of the associated entity if the entity property is an association (either a @OneToOne or a @ManyToOne).
As you can see the last case is far from obvious. It has been inherited from the dark ages of EJB 2 for backward compatibilities and we recommend you not to use it (for simplicity sake).

Let's explore all three cases using examples.

That two level nested PK is not working.