MongoDB聚合框架中的按数字分组

Question

我试图使用以下管道计算在特定年份出生的用户的数量。

{"$group": {"_id": "$birthdayYear", "count": {"$sum": 1}}}

但是我得到的是，所有年份都是sum，而Mongo没有$count函数，那么我能做些什么来根据用户出生的年份对用户进行分组呢？

示例文档

{
    "_id" : "LrN2tAK4rtyhsYFbr",
    "firstName" : "abc",
    "lastName" : "",
    "birthdayDay" : 5,
    "birthdayMonth" : 8,
    "birthdayYear" : 1984,
}

/* 1 */

{
    "_id" : "birthDayYear",
    "count" : 5550.0
}

Answer 1

我已经将两个文档插入到用户集合中。

db.users.insert({ "_id" : "LrN2tAK4rtyhsYFbs",     "firstName" : "test",     "lastName" : "1",     "birthdayDay" : 5,     "birthdayMonth" : 8,     "birthdayYear" : 1984})

db.users.insert({ "_id" : "LrN2tAK4rtyhsYFbr",     "firstName" : "test",     "lastName" : "2",     "birthdayDay" : 6,     "birthdayMonth" : 9,     "birthdayYear" : 1984})

并运行以下查询

db.users.aggregate([{$group: {"_id": "$birthdayYear", count: {$sum:1}} }])

得到了结果

{ "_id" : 1984, "count" : 2 }

Answer 2

请检查聚合语句的语法--我相信下面显示了所需的结果：

 db.tstso1.aggregate([{$group : {_id : "$birthdayYear", count: { $sum: 1 }}}])

结果：

{ "_id" : 1986, "count" : 2 }
{ "_id" : 1985, "count" : 3 }
{ "_id" : 1984, "count" : 6 }

文档：

 see.pretty();
{
    "_id" : "LrN2tAK4rtyhsYFbr",
    "firstName" : "abc",
    "lastName" : "",
    "birthdayDay" : 5,
    "birthdayMonth" : 8,
    "birthdayYear" : 1984
}
{
    "_id" : "2",
    "firstName" : "abc",
    "lastName" : "",
    "birthdayDay" : 5,
    "birthdayMonth" : 8,
    "birthdayYear" : 1984
}
{
    "_id" : "3",
    "firstName" : "abc",
    "lastName" : "",
    "birthdayDay" : 5,
    "birthdayMonth" : 8,
    "birthdayYear" : 1984
}
{
    "_id" : "4",
    "firstName" : "abc",
    "lastName" : "",
    "birthdayDay" : 5,
    "birthdayMonth" : 8,
    "birthdayYear" : 1984
}
{
    "_id" : "6",
    "firstName" : "abc",
    "lastName" : "",
    "birthdayDay" : 5,
    "birthdayMonth" : 8,
    "birthdayYear" : 1984
}
{
    "_id" : "7",
    "firstName" : "abc",
    "lastName" : "",
    "birthdayDay" : 5,
    "birthdayMonth" : 8,
    "birthdayYear" : 1984
}
{
    "_id" : "12",
    "firstName" : "abc",
    "lastName" : "",
    "birthdayDay" : 5,
    "birthdayMonth" : 8,
    "birthdayYear" : 1985
}
{
    "_id" : "13",
    "firstName" : "abc",
    "lastName" : "",
    "birthdayDay" : 5,
    "birthdayMonth" : 8,
    "birthdayYear" : 1986
}
{
    "_id" : "14",
    "firstName" : "abc",
    "lastName" : "",
    "birthdayDay" : 5,
    "birthdayMonth" : 8,
    "birthdayYear" : 1985
}
{
    "_id" : "16",
    "firstName" : "abc",
    "lastName" : "",
    "birthdayDay" : 5,
    "birthdayMonth" : 8,
    "birthdayYear" : 1986
}
{
    "_id" : "17",
    "firstName" : "abc",
    "lastName" : "",
    "birthdayDay" : 5,
    "birthdayMonth" : 8,
    "birthdayYear" : 1985
}