我們如何計算mongodb java中不同文檔中arraylist的項目數量？

Question

假設我有這些文件：

{ "_id" : 1, "item" : "ABC1", "description" : "product 1", colors: [ "blue", "black", "red" ] } 
{ "_id" : 2, "item" : "ABC2", "description" : "product 2", colors: [ "blue", "purple" ] } 
{ "_id" : 3, "item" : "XYZ1", "description" : "product 3", colors: [ "red", "yellow"] } 

結果我想是

blue : 2 
red : 2 
black : 1 
purple : 1 
yellow : 1 

這可能使用MongoDB的執行或者我需要得到顏色數組中的Java後手動執行？提示或任何幫助將非常有用。

這是我如何得到數據庫和集合

MongoClient mongoClient = new MongoClient("localhost", 27017); 
MongoCollection<Document> collection = mongoClient.getDatabase("tweets") 
      .getCollection("tweet"); 

Answer 1

你可以試試下面的聚集。

db.collection.aggregate({$unwind:"$colors"},{$group: { _id:"$colors",count:{$sum:1} }}) 

該查詢將$unwind該數組。來自文檔

解構輸入文檔中的數組字段以輸出每個元素的 文檔。

$unwind階段後的響應。

{ "_id" : 1, "item" : "ABC1", "description" : "product 1", "colors" : "blue" } 
{ "_id" : 1, "item" : "ABC1", "description" : "product 1", "colors" : "black" } 
{ "_id" : 1, "item" : "ABC1", "description" : "product 1", "colors" : "red" } 
{ "_id" : 2, "item" : "ABC2", "description" : "product 2", "colors" : "blue" } 
{ "_id" : 2, "item" : "ABC2", "description" : "product 2", "colors" : "purple" } 
{ "_id" : 3, "item" : "XYZ1", "description" : "product 3", "colors" : "red" } 
{ "_id" : 3, "item" : "XYZ1", "description" : "product 3", "colors" : "yellow" } 

下一步是$group上color的數據，而使用$sum計數的顏色

最終輸出

{ "_id" : "yellow", "count" : 1 } 
{ "_id" : "blue", "count" : 2 } 
{ "_id" : "black", "count" : 1 } 
{ "_id" : "red", "count" : 2 } 
{ "_id" : "purple", "count" : 1 } 

的Java更新：

3.x版

collection.aggregate(Arrays.asList(Aggregates.unwind("$colors"), Aggregates.group("$colors", Accumulators.sum("count", 1)))); 

2.x的版

collection.aggregate(Arrays.asList(new BasicDBObject("$unwind", "$colors"), new BasicDBObject("$group", new BasicDBObject("_id","$colors").append("count",new BasicDBObject("$sum", 1)))));