統計整個MongoDB集合中數組中唯一值的出現次數

Question

因此，我有一組用戶。用戶文件是一個非常簡單的文件，如下所示：

{ 
    username: "player101", 
    badges: ["score10", "score100"] 
} 

所以，我怎麼可以查詢看到徽章陣列中的每個獨特的價值有多少次在整個回收時？

Answer 1

使用aggregation與$unwind和$group階段，在那裏你可以$sum算術運算符

db.players.aggregate([ 
    { 
    $unwind: "$badges" 
    }, 
    { 
    $group: 
     { 
     _id: "$badges", 
     count: { $sum: 1 } 
     } 
    } 
]); 

上收集players與文件總結徽章

{ "username" : "player101", "badges" : [ "score10", "score100" ] } 
{ "username" : "player102", "badges" : [ "score11", "score100" ] } 
{ "username" : "player103", "badges" : [ "score11", "score101" ] } 
{ "username" : "player104", "badges" : [ "score12", "score100" ] } 

給你結果

{ "_id" : "score101", "count" : 1 } 
{ "_id" : "score11", "count" : 2 } 
{ "_id" : "score12", "count" : 1 } 
{ "_id" : "score100", "count" : 3 } 
{ "_id" : "score10", "count" : 1 }