所以我在mongo中有这个测试数据，Cart模型:{"_id":ObjectId("55eb513c516ddc8fa6e68886"),"user":ObjectId("55e3f236516ddc78296968be"),"items":[{"item":ObjectId("55eb10b8516ddc7508dba7c2"),"quantity":1,"added_date":ISODate("2015-09-05T20:32:16.527Z"),"coupons":ObjectId("55eb10cd516ddc751d3d5e25"),"order_type":["in_st