simplify mongo query by using $setOnInsert, requires mongo 2.6
simplify mongo query by using $setOnInsert, requires mongo 2.6

guardian/alerta,mrkeng/alerta,skob/alerta,mrkeng/alerta,0312birdzhang/alerta,guardian/alerta,skob/alerta,0312birdzhang/alerta,mrkeng/alerta,0312birdzhang/alerta,guardian/alerta,mrkeng/alerta,skob/alerta,skob/alerta,guardian/alerta