MongoDB Java 错误：管道阶段规范对象必须恰好包含一个字段

问题描述

我正在尝试在 java 上获取此 MongoDB 查询的结果。

db.filetree.aggregate([
    {
        $match: {
            "_id": "6062144bb25e4809548ef246",}
    },{
        $unwind: "$children"
    },{
        $match: {
            "children.fileName": "Test1"
        }
    },{
        $project: {
            "_id": 0,"fileId": "$children.fileId","fileName": "$children.fileName","directory": "$children.directory",}
    }
]).pretty()

查询运行良好，在没有数据时不显示任何内容。但是，从 java 执行的查询会产生以下错误：

com.mongodb.MongoCommandException: Command Failed with error 40323 (Location40323): 'A pipeline stage specification object must contain exactly one field.' on server localhost:27017. The full response is {"ok": 0.0,"errmsg": "A pipeline stage specification object must contain exactly one field.","code": 40323,"codeName": "Location40323"}

ChildFile findChildInParent(String parentId,String fileName) {
        BasicDBObject idFilter = new BasicDBObject().append("_id",parentId);
        BasicDBObject matchId = new BasicDBObject().append("$match",idFilter);
        BasicDBObject unwindChildren = new BasicDBObject().append("$unwind","$children");
        BasicDBObject childNameFilter = new BasicDBObject().append("children.fileName",fileName);
        BasicDBObject matchChildName = new BasicDBObject().append("$match",childNameFilter);
        BasicDBObject projections = new BasicDBObject()
                .append("_id",0)
                .append("fileId","$children.fileId")
                .append("fileName","$children.fileName")
                .append("directory","$children.directory");

        List<ChildFile> childFiles = fileCollection.aggregate(
                List.of(matchId,unwindChildren,matchChildName,projections),ChildFile.class
        ).into(new ArrayList<>());

        return childFiles.size() > 0 ? childFiles.get(0) : null;
    }

我在这里遗漏了什么吗？任何帮助都非常感谢。谢谢?！

解决方法

您的代码有错别字，您缺少 $ 字段的 children

应该是：

BasicDBObject unwindChildren = new BasicDBObject().append("$unwind","$children")

代替：

BasicDBObject unwindChildren = new BasicDBObject().append("$unwind","children")

也缺少 $poject 阶段：

BasicDBObject projections = new BasicDBObject()
        .append("_id",0)
        .append("fileId","$children.fileId")
        .append("fileName","$children.fileName")
        .append("directory","$children.directory");

BasicDBObject projectionStage = new BasicDBObject().append("$project",projections);

List<ChildFile> childFiles = fileCollection.aggregate(
        List.of(matchId,unwindChildren,matchChildName,projectionStage),ChildFile.class
).into(new ArrayList<>());