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相关的任务。我正在尝试用Java创建一个MongoDB查询，该查询将返回Task并嵌入与其链接的Project，但是到目前为止我完成的代码只返回一个空数组（我有将此something命名为下面的[]。

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

我使用的mongo-java-driver版本3.5.0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();
}

此方法目前返回以下内容：

{
    _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
    }
}

以下是MongoDB中存储的数据的示例：

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。

但是，有一个workaround to transform DBRefs into simple ObjectId that I have detailed on a different answer，你可以用Java调整它或在原生查询中使用它。

MongoDB $查询查询返回空字段

1 个答案: