我们如何计算来自mongodb java中不同文档的arraylist中的项目数？

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"] }

我想要的结果是

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

这是否可以使用 mongodb 执行，或者我需要在 Java 中获取颜色数组后手动实现？提示或任何帮助都会非常有用。

这就是我获取数据库和集合的方式

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合。

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

查询将$unwind 数组。来自文档

从输入文档中解构一个数组字段以输出一个 每个元素的文档。

$unwind 阶段之后的响应。

{ "_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$groupcolor 上的数据，同时使用$sum 计算颜色

最终输出

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

Java 更新：

3.x 版本

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

2.x 版本

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