Codeigniter migration FOREIGN KEY

class Migration_Add_f_k_user_friends extends CI_Migration {

    public function __construct()
    {
        $this->load->database();
    }

    public function up() {
        $this->db->query('ALTER TABLE friends
                            ADD CONSTRAINT fk_UserFriends
                            FOREIGN KEY (user_id)
                            REFERENCES user(id)');
        $this->db->query('ALTER TABLE friends
                            ADD CONSTRAINT fk_FriendsUser
                            FOREIGN KEY (friend_id)
                            REFERENCES user(id)');
    }

    public function down() {
        $this->db->query('ALTER TABLE friends
                            DROP FOREIGN KEY fk_UserFriends');
        $this->db->query('ALTER TABLE friends
                            DROP FOREIGN KEY fk_FriendsUser');
    }

}