Mongodb找到每个组中最新的并在spring boot实现

Question

我是MongoDB的新手。假设我有一个集合中的文档：

  [
        {
          _id: ObjectId("1"),
          uid: "test",
          obj: Object
                    name: "a",
                    field1: "..."
        },

        {
          _id: ObjectId("2"),
          uid: "test",
          obj: Object
                    name: "a",
                    field1: "..."
        },
        {
          _id: ObjectId("3"),
          uid: "test",
          obj: Object
                    name: "b",
                    field1: "..."
        },
         {
          _id: ObjectId("4"),
          uid: "test2",
          obj: Object
                    name: "b",
                    field1: "..."
        }
    ]

我想检索具有唯一obj.name的obj列表，预期输出为：

  [
      {
         _id: ObjectId("2") // not really necessary , just to show that the obj was under ObjectId("2")
         name: "a",
         field1: "..."
      },
      {
         _id: ObjectId("3") // not really necessary , just to show that the obj was under ObjectId("3")
         name: "b",
         field1: "..."
      }
    ]

我想做到这一点：

>

首先匹配uid

按_id desc对结果排序

按对象.名称分组

[{$match:{uid：“test”}},{$sort:{_id:-1}},{$group:{_id:“$obj.name”}}]

我得到的是：

{
  _id:"a"
},
{
  _id:"b"
}

第二个问题：如何用spring boot mongo模板或其他spring boot mongo库进行这样的查询？

 Public class A {
       private String _id;
       private String uid;
       private Obj obj;
    }

    Public class Obj {
           private String name;
           private String field1;
    }

在Java意义上，我想检索列表

但不知道怎么做。

非常感谢你的帮助。

Answer 1

这很简单。按照教程操作。

添加以下代码：

//Add this code to your service class
@Autowired
private MongoTemplate mongoTemplate;

...

//Aggregation pipeline    
Aggregation agg = Aggregation.newAggregation(
    Aggregation.match(Criteria.where("uid").is("test")),
    Aggregation.sort(Direction.DESC, "_id"),
    Aggregation.group("obj.name").first("obj").as("obj"),
    Aggregation.replaceRoot("obj")
);

AggregationResults<Obj> result = mongoTemplate.aggregate(agg, "collection", Obj.class).getMappedResults();
//result.forEach(System.out::println);