MY_Model $before_get example.

//In Model:

class some_model extends MY_Model {
    protected $before_get;

    public function get_by_column()
    {
        $before_get = $this->db->select('username');
        $query = $this->some_model->get_all();
        return $query;
    }
}

//In Controller
$this->some_model->get_by_column()

//of course a better example would be to pass an array to get_by_column...  :)