Zkuste toto, možná to budete muset mírně změnit:
function get_job(){
$data = array();
$data = $this->db->get('job')->result_array();
foreach( $data as $key=>$each ){
$data[$key] = $this->db->where('idadministrator', $each['idadministrator'])->get('administrator')->row_array();
$data[$key]['artwork'] = $this->db->where('idjob', $each['idjob'])->get('artwork')->result_array();
}
return $data;
}