MongoDb如何按月和年汇总

Question

我是mongodb的新手并且正在尝试学习Mongodb查询

    {
    "_id" : ObjectId("59815d4704ca1760a45957ca"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 200,
    "expenseRemark" : "aa",
    "expenseCategory" : "billing",
    "entryTime" : ISODate("2017-08-02T05:03:57Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("59815d5404ca1760a45957cb"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 300,
    "expenseRemark" : "ff",
    "expenseCategory" : "transport",
    "entryTime" : ISODate("2017-08-02T05:04:11Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("5980191d04ca1760a45957cd"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 100,
    "expenseRemark" : "rr",
    "expenseCategory" : "billing",
    "entryTime" : ISODate("2017-08-01T06:00:46Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("5980192604ca1760a45957ce"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 200,
    "expenseRemark" : "qq",
    "expenseCategory" : "transport",
    "entryTime" : ISODate("2017-08-01T06:01:03Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("5980192e04ca1760a45957cf"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 470,
    "expenseRemark" : "ff",
    "expenseCategory" : "transport",
    "entryTime" : ISODate("2017-08-01T06:01:11Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("59816ac004ca1760a45957d0"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 500,
    "expenseRemark" : "raj",
    "expenseCategory" : "transport",
    "entryTime" : ISODate("2017-08-02T06:01:26Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("59816acb04ca1760a45957d1"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 100,
    "expenseRemark" : "pet",
    "expenseCategory" : "pets",
    "entryTime" : ISODate("2017-08-02T06:01:37Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("597d7a9c04ca1760a45957d2"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 500,
    "expenseRemark" : "gt",
    "expenseCategory" : "sports",
    "entryTime" : ISODate("2017-07-30T06:20:04Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("597d7aaa04ca1760a45957d3"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 560,
    "expenseRemark" : "mov",
    "expenseCategory" : "entertainment",
    "entryTime" : ISODate("2017-07-30T06:20:14Z"),
    "__v" : 0
}

我想得到按年+月分组的费用.我试过了

`db.expenses.aggregate( 
       {$project : { 
              month : {$month : "$entryTime"}, 
              year : {$year :  "$entryTime"}
          }}, 
        {$group : { 
                _id : {month : "$month" ,year : "$year" },  
              total : {$sum : "$expenseAmount"} 
        }})`

这使

{"_ id":{"month":7,"year":2017},"total":0}

{"_ id":{"month":8,"year":2017},"total":0}

请指导我如何获得汇总结果.我无法弄清楚这样做的方法.

谢谢

Answer 1

你expenseAmount在投影阶段失去了领域.只需添加它:

   {$project : { 
          month : {$month : "$entryTime"}, 
          year : {$year :  "$entryTime"},
          expenseAmount : 1
      }},

请注意,如果文档中不存在该字段,则$sum返回0.

另请注意,还有另一个聚合运算符,它可以按预期执行 - $addFields.它添加了新字段以记录和保留输入文档中的所有现有字段.但在这种情况下,您只需要expenseAmount字段