Re-using a JoinTable in grails

Question

I have two domains that have a many-to-many relationship. Example:

class LibraryUser {
    String name
    static hasMany = [ checkedoutBooks : Book ]
}

class Book {
    String title
    static hasMany = [ usersWhoCheckedItOut : LibraryUser ]
}

However I now want to record the time of checkout and store this in the already existing join table:

class Checkouts {
    LibraryUser  user
    Book book

    Date dateCreated
}

class LibraryUser {
    String name
    static hasMany = [ checkedoutBooks : Book ]
    static mapping = {
        checkedoutBooks joinTable: [
            name:'checkouts',
            key:'libraryuser_id',
            column:'book_id'
        ]
    }
}

class Book {
    String title
    static hasMany = [ usersWhoCheckedItOut : LibraryUser ]
    static mapping = {
        usersWhoCheckedItOut joinTable: [
            name:'checkouts',
            key:'book_id',
            column:'libraryuser_id'
        ]
    }
}

However what I get is a checkouts table where the id column is not set to auto_increment (instead the libraryuser_id is set to auto_increment which is definitely not making any sense in my case). I suspect there is a bug somewhere in my above code?