按月mongodb查询汇总数据

Question

我有一个mongodb：

    {
    "_id" : ObjectId("59816ac004ca1760a45957d0"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 500,
    "expenseRemark" : "raj",
    "expenseCategory" : "transport",
    "transactionType":"debit",
    "entryTime" : ISODate("2017-08-02T06:01:26Z"),
    "__v" : 0
},
{
    "_id" : ObjectId("59816acb04ca1760a45957d1"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 100,
    "expenseRemark" : "pet",
    "expenseCategory" : "pets",
    "transactionType":"debit",
    "entryTime" : ISODate("2017-08-02T06:01:37Z"),
    "__v" : 0
},
{
    "_id" : ObjectId("597d7a9c04ca1760a45957d2"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 500,
    "expenseRemark" : "gt",
    "expenseCategory" : "sports",
    "transactionType":"debit",
    "entryTime" : ISODate("2017-07-30T06:20:04Z"),
    "__v" : 0
},
{
    "_id" : ObjectId("597d7aaa04ca1760a45957d3"),
    "userEmail" : "lk@gmail.com",
    "expenseAmount" : 560,
    "expenseRemark" : "mov",
    "expenseCategory" : "entertainment",
    "transactionType":"debit",
    "entryTime" : ISODate("2017-07-30T06:20:14Z"),
    "__v" : 0
}

我希望结果能够给出7月份expenseAmount的总和。我怎么能实现这一点，到目前为止我已经尝试过了，

`

db.expenses.aggregate(
  [ 
    { 
      $match:{"userEmail":"lk@gmail.com","entryTime":{"$month": 7}}
    }, 
    {$group:{ _id : null, total : {$sum : "$expenseAmount"}}} 
  ]
)

`

但它不起作用。请指导我，我该怎么做。我做了各种各样的事情但没有工作。虽然我可以通过在entryTime中指定$lt & $gt来获得结果

RGDS