Something like this:

You have a list of ids, and instead of fetching all records of them, you'll want to set a relation from a record to this list of ids.

Example (prototype syntax):

$this->user->has_many('posts');
$record = $this->user->find(1);

$list = array(1, 2, 3, 4, 5);  // post ids of the records to relate to

$record->add('posts', $list);