Problem migrating Entity Reference fields with Drupal 7

Question

I cannot migrate entity reference fields Migrate . I am following the examples online i.e.

$this->addFieldMapping('my_reference_field','bla')->sourceMigration('BooksNode')->separator(',');

The idea is for a book lending data. I can lend several books to one person (not a user on my site, just a regular content type).

Not sure what I am missing but nothing gets mapped.

I should add that the primary key on my Books table (in MySQL) is a composite one consisting of two fields id and unique_id. Do I need to add that somehow to :

$this->addFieldMapping('my_reference_field','bla')->sourceMigration('BooksNode')->separator(',');

maybe

$this->addFieldMapping('my_reference_field',array('id', 'unique_id'))->sourceMigration('BooksNode')->separator(',');

thx in advance