我们如何计算mongodb java中不同文档中arraylist的项目数量？

Question

假设我有这些文件：

{ "_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"] } 

结果我想是

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

这可能使用MongoDB的执行或者我需要得到颜色数组中的Java后手动执行？提示或任何帮助将非常有用。

这是我如何得到数据库和集合

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

Answer 1

你可以试试下面的聚集。

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

该查询将$unwind该数组。来自文档

解构输入文档中的数组字段以输出每个元素的 文档。

$unwind阶段后的响应。

{ "_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$group上color的数据，而使用$sum计数的颜色

最终输出

{ "_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)))));