Grails/GORM: How can I create a composite primary key with two foreign keys?

[msarto]

I'm hoping someone can shed some light on how to declare the "BookShelf" class I'm trying to create in Grails/GORM. I have the following simple domain objects:

class Book {
     int      bookID
     String title
     String author

     // static mapping assigns primary key to bookID
}

class Shelf {
    int shelfID
    String name

   // static mapping assigns primary key to shelfID
}

What I want to do is create a "BookShelf" domain where I can set the number of book copies on a particular book shelf, such as follows:

class BookShelf {
    int bookID    // linked to "Book.bookID"
    int shelfID    // linked to "Shelf.shelfID"
    int bookCopies

    // bookID and shelfID would be a composite unique primary key constrained by an existing bookID and shelfID
}

I'd like the BookShelf primary key to be a unique composite of Book.bookID and Shelf.shelfID -- in order to be able to create a Shelf that has multiple copies of a "Book".

I'd appreciate any help anyone could give on this, or even a different way to approach this relationship in Grails.