如何使用聚合管道按兩級分組進行排序？

Question

鑑於這樣的數據：

[ {country:"US", city:"NY", cnt: 10} 
    {country:"IT", city:"MI", cnt: 9} 
    {country:"US", city:"LA", cnt: 8} 
    {country:"IT", city:"RM", cnt: 20} ] 

是否有使用MongoDB的聚合管道創建一個結果數組，看起來像這樣（不基於阿爾法碼，只是CNT）的方式：

[ {country:"IT", city:"RM", cnt:20} 
    {country:"IT", city:"MI", cnt: 9} 
    {country:"US", city:"NY", cnt:10} 
    {country:"US", city:"LA", cnt: 8} ] 

}

換句話說，一個數組按照總數最高的國家排序（降序），然後按每個城市的最高總數排序（降序）？

我可以按國家或國家和城市分組，但都不會給我上述結果。每個國家給我兩條總計，另一條給我四條國家城市總數，但沒有按總數最高的國家排序。在

{ "$sort": { "country": 1, "cnt": -1 } } 

結果：

Answer 1

就在$group之後添加$sort

{ "country" : "IT", "city" : "RM", "cnt" : 20 } 
{ "country" : "IT", "city" : "MI", "cnt" : 9 } 
{ "country" : "US", "city" : "NY", "cnt" : 10 } 
{ "country" : "US", "city" : "LA", "cnt" : 8 } 

要使用總計然後小組獲得總計數：

{ "$group": { 
    "_id": "$country", 
    "cities": { "$push": { 
    "city": "$city", 
    "cnt": "$cnt" 
    }}, 
    "totalCount": { "$sum": "$cnt" } 
}}, 
{ "$unwind": "$cities" }, 
{ "$sort": { "totalCount": -1, "_id": 1, "cities.cnt": -1 }}, 
{ "$project": { 
    "_id": 0, 
    "country": "$_id", 
    "city": "$cities.city", 
    "cnt": "$cities.cnt" 
}} 

項目爲了得到s ame結果