使用带有投影的find()方法使用mongodb java driver 3.4检索数据 [英] retrieving data with mongodb java driver 3.4 using find()-method with projection

问题描述

我正在使用mongodb java驱动程序3.4.

I am using mongodb java driver 3.4.

在mongodb数据库中，文件根据以下结构保存:

In the mongodb database documents are saved according to the following structure:

{
    "_id" : ObjectId("595a9fc4fe3f36402b7edf0e"),
    "id" : "123",
    "priceInfo" : [
        {object1: value1}, {object2: value2}, {object3: value3}
    ]
}

为了检索具有特定ID的文档的"priceInfo"-数组，我编写了以下代码:

In order to retrieve the "priceInfo"-Array of a Document with a specific id, I wrote the following code:

collection.find(eq("id", id)).first().projection(fields(include("priceInfo"), excludeId()));

我也是根据文档编写此代码的，您可以在这里找到:

I wrote this code according too the documentation, which you can find here:

http: //mongodb.github.io/mongo-java-driver/3.4/javadoc/?com/mongodb/client/model/Projections.html

问题是我的IDE不接受此代码.

The problem is that my IDE won't accept this code.

它给我以下错误指示:

我不知道为什么这段代码不起作用.最初，IDE建议包括几个类，而我确实做到了.但是之后，我仍然收到错误指示，即上面看到的错误指示.

I have no clue why this code doesn't work. At first the IDE suggested including several classes - which I did. But after that I still got an error indication, namely the one you see above.

代码有什么问题?如何检索ID为ID的文档的priceInfo数组?

What's wrong with the code? How can I retrieve the priceInfo array of a Document with ID id?

********************************更新************** ********************

********************************UPDATE**********************************

根据请求，这是整个课程:

As per request, here's the whole class:

package DatabaseAccess;

import Models.GasStation;
import com.mongodb.MongoClient;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import static com.mongodb.client.model.Filters.eq;
import static com.mongodb.client.model.Projections.excludeId;
import static com.mongodb.client.model.Projections.fields;
import static com.mongodb.client.model.Projections.include;
import com.mongodb.client.model.Updates;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Date;
import java.util.logging.Level;
import org.bson.Document;


public class databaseAccess {

    private final String DB_HOST = "localhost"; 
    private final int DB_PORT = 27017;
    private final String DB_NAME = "db1"; 
    private final String DB_COLLECTION = "prices"; 
    private final MongoClient mongoClient;
    private final MongoDatabase database;
    private final MongoCollection<Document> collection; 

    public databaseAccess(){
        mongoClient = new MongoClient(DB_HOST, DB_PORT);
        database = mongoClient.getDatabase(DB_NAME);
        collection = database.getCollection(DB_COLLECTION);
    }


    public String readFromDB(String id){
        collection.find(eq("id", id)).first().projection(fields(include("priceInfo"), excludeId()));
        return null;     
    }

}

推荐答案

您正在对方法中的调用链进行操作. 让我们分析链中的每个元素:

You're operating on chain of calls in your method. Let's analyze each element in chain:

MongoCollection:

MongoCollection:

FindIterable< TDocument> find()-查找集合中的所有文档.

FindIterable< TDocument> find() - Finds all documents in the collection.

返回类型为FindIterable<TDocument>，您在其上调用链中的下一个方法:

Return type is FindIterable<TDocument> and you calling the next method in chain on it:

FindIterable< TDocument>

FindIterable< TDocument>

从com.mongodb.async.client.MongoIterable接口继承的方法:

Methods inherited from interface com.mongodb.async.client.MongoIterable:

batchCursor，首先是forEach，进入地图

batchCursor, first, forEach, into, map

好的，我们要去MongoIterable:

MongoIterable< TResult>:

MongoIterable< TResult>:

void first(SingleResultCallback回调)-帮助程序返回迭代器中的第一项或为null.

void first(SingleResultCallback callback) - Helper to return the first item in the iterator or null.

这意味着first(...)不返回任何内容.您是从零开始调用projection(...)的，当然这是不适用的，因此编译器将其标记为错误.

That means first(...) is returned nothing. You're calling projection(...) from nothing, of course this is not applicable, so the compiler marks this as an error.

要调用projection(Bson projection)，您应该拥有FindIterable<T>实例. MongoCollection.find()可以为您提供此实例:

For calling projection(Bson projection) you shoud have FindIterable<T> instance. MongoCollection.find() can provide you with this instance:

collection.find(eq("id", id)).projection(fields(include("priceInfo"), excludeId()));

这篇关于使用带有投影的find()方法使用mongodb java driver 3.4检索数据的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！