mongodb：如何按百分比计算月差会话

Question

我的收藏集metrics包含以下数据：

{ "year" : "2011", "month" : "01", "day" : "1", "sessions" : 10 }
{ "year" : "2011", "month" : "01", "day" : "2", "sessions" : 20 }
{ "year" : "2011", "month" : "01", "day" : "3", "sessions" : 30 }
......
{ "year" : "2011", "month" : "02", "day" : "19", "sessions" : 10 }
......
{ "year" : "2011", "month" : "03", "day" : "20", "sessions" : 100 }
...........

我想知道每个月sessions的百分比与上个月相比有所增加或减少？

预期结果示例

2011-01  100%
2011-02  -83% (= (10-60)/60 )
2011-03  +900% (= (100-10)/10 ) 

是否可以在mongodb查询中执行此操作？