I have two models, called Book and Tag, which are in a HABTM relationship. I want a couple (book, tag) to be saved only once. In my models I have

var $hasAndBelongsToMany = array(
    'Tag' => array(
        'className' => 'Tag',
        'joinTable' => 'books_tags',
        'foreignKey' => 'book_id',
        'associationForeignKey' => 'tag_id',
        'unique' => true
    )
);

and viceversa, but the Unique flag does not help me; I can still save two times the same couple.

How do I do this in CakePHP? Should I declare the couple (book, tag) unique in the database directly, or will this make CakePHP go nuts? Is there a Cakey way to handle this situation?

EDIT: I tried making the couple unique with the query (I'm using MySQL)

ALTER TABLE books_tags ADD UNIQUE (book_id,tag_id);

but this does not work well. When I save more than one tag at a time, everything goes well if all the couples are new. If at least one of the couples is repeated, CakePHP fails to do the whole operation, so it does not save ANY new couple (not even the good ones).