使用Java中的$ elemMatch实现Mongodb查询

Posted 2021-04-16

我使用Mongo java驱动程序从mongo集合中检索数据。我有以下查询，我试图在java中实现：Json是：

{
    "_id" : ObjectId("56cd284767c74d3d4dd3ec80"),
    "comments" : "Hello",
    "statusLog" : [ 
        {
            "status" : "Submitted",
            "startDate" : ISODate("2015-01-14T05:00:00.000Z"),
            "endDate" : ISODate("2016-02-29T21:24:24.740Z")
        }, 
        {
            "status" : "Active",
            "startDate" : ISODate("2016-02-29T21:24:24.740Z")
        }
    ],
    "createdDate" : ISODate("2015-01-14T05:00:00.000Z")
}

Mongo查询：

db.CollectionName.find({},{_id: 0, createdDate:1, "statusLog": {$elemMatch: {"status":"Submitted"}}});

这是我编写的java中的查询（mongo java driver 3.4.2）：

BasicDBObject query = new BasicDBObject(new BasicDBObject("statusLog",
                new BasicDBObject("$elemMatch", new BasicDBObject("status", "Submitted"))));

运行java代码将返回所有状态日志，而不是我要查找的日志。

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答案

您应该使用较新的Document/MongoCollection api并使用辅助方法来准备投影。

import static com.mongodb.client.model.Filters.eq;
import static com.mongodb.client.model.Projections.*;

MongoClient mongoClient = new MongoClient(); 
MongoDatabase database = mongoClient.getDatabase("db"); // Get DB

MongoCollection<Document> collection = database.getCollection("collection"); // Get Collection
Bson projection = Projections.fields( excludeId(), include("createdDate"), elemMatch("statusLog", eq("status", "Submitted"))); // Add Projections
FindIterable<Document> iterable = collection.find().projection(projection);

使用旧的BasicDBObject/DBCollection api

MongoClient mongoClient = new MongoClient();
DB database = mongoClient.getDB("db");

DBCollection collection = database.getCollection("collection");
BasicDBObject projection = new BasicDBObject(new BasicDBObject("statusLog",new BasicDBObject("$elemMatch", new BasicDBObject("status", "Submitted"))));
collection.find(new BasicDBObject(), projection);

另一答案

接受的答案是错误的。 elemMatch应该在find方法中而不是在投影bson中。例如：

collection.find(elemMatch("statusLog", eq("status", "Submitted"))).projection(projection);