java - MongoDB $lookup 查询返回空字段

Question

我的数据库中有两个集合，Projects 和 Tasks。每个项目都可以有许多与之关联的任务。我正在尝试在 Java 中创建一个 MongoDB 查询，它将返回一个 Task 并嵌入与之链接的 Project，但是我到目前为止完成的代码只返回一个空数组(我将此命名为 something，如下面的 [] 所示)。

不幸的是，我没有找到很多关于如何在 Java 中正确使用 $lookup 的示例。我需要对代码执行哪些更改才能使 something 字段返回 project？我应该使用 $lookup 还是其他聚合运算符？

我使用 mongo-java-driver 3.5.0 版的 Java 方法:

public static String readTask()
{
    MongoClient mongoClient = new MongoClient();
    MongoDatabase database = mongoClient.getDatabase("exampleDatabase");

    Bson lookup = new Document("$lookup",
            new Document("from", "Project")
                    .append("localField", "project._id")
                    .append("foreignField", "_id")
                    .append("as", "something"));

    List<Bson> filters = new ArrayList<>();
    filters.add(lookup);

    AggregateIterable<Document> it = database.getCollection("Task").aggregate(filters);
    System.out.println("First Document: " + it.first().toString());

    return it.first().toString();
}

此方法当前返回以下内容:

{
    _id = 599a62cac29d9a2684c64012,
    constructionRoomNumber = 15,
    type = Example,
    summary = Summary Text,
    description = Description Text,
    status = Open,
    project = {
        "$ref": "Project",
        "$id": "5996582a0983347784fb2ff4"
    },
    something = []
}

预期结果:

{
    _id: ObjectId('599a62cac29d9a2684c64012')
    constructionRoomNumber: "15"
    type: "Example"
    summary: "Summary Text"
    description: "Description Text"
    status: "Open"
    project: {
        _id: ObjectId('5996582a0983347784fb2ff4')
        projectCode: "V1000"
        projectName: "Example Project"
        projectLocation: "1 Somewhere Street, Some City"
        clientName: "Whatever Client"
        isActive: true
    }
}

这是存储在 MongoDB 中的数据的示例:

示例项目:

_id: ObjectId('5996582a0983347784fb2ff4')
projectCode: "V1000"
projectName: "Example Project"
projectLocation: "1 Somewhere Street, Some City"
clientName: "Whatever Client"
isActive: true

示例任务:

_id: ObjectId('599a62cac29d9a2684c64012')
constructionRoomNumber: "15"
type: "Example"
summary: "Summary Text"
description: "Description Text"
status: "Open"
project: DBRef(Project, 5996582a0983347784fb2ff4, undefined)

Answer 1

您正在尝试查找不受支持的 DBRef 字段。

然而，有一个workaround to transform DBRefs into simple ObjectId that I have detailed on a different answer ，您可以在 Java 中对其进行调整或在 native 查询中使用它。