如何在mongoDB上进行此查询？

Question

这是我的收藏结构......

   {
       "_id": ObjectId("5316f53228551c0af600019b"),
       "age_range": {
         "min": NumberInt(21)
      },
 "likes": {
     "data": {
       "0": {
         "category": "Tv show",
         "name": "Those Who Kill on A&E",
         "created_time": "2014-03-03T15:08:02+0000",
         "id": "629689387051864"
      },
       "1": {
         "category": "Journalist",
         "name": "Robin Roberts",
         "created_time": "2014-02-24T18:02:43+0000",
         "id": "780950061932847"
      }
},
{
 "_id": ObjectId("5316f53228551c0af600019b"),
`

我需要获取每个类别的用户总数。 例如

Category Name: Journalist
Total users: 10 

Answer 1

试试这个（集合应该是你拥有的集合）

var mapFunction = function() {
    for (a in this.likes.data){
        var name = this.likes.data[a].category;
        emit(name, 1);
    }
}
var reduceFunction = function(key, val){
    return Array.sum(val);
}
db.collection.mapReduce(mapFunction, reduceFunction, {out: "myresult"});
db.myresult.find();