If the user wants it (ie. specifies it as a setting on the relation), join it in automatically.

This will also make this example more neat: http://codeigniter.com/forums/viewreply/467797/

$pages_by_title = $art->related('associations')->order_by('page_title')->get();
// instead of
$pages_by_title = $art->related('associations')->join_related('page')->order_by('page_title')->get();