java - 使用 Java 驱动程序运行聚合 - MongoDB

Question

使用我使用的 MongoDB shell:

db.bios.aggregate(
[
{$match:{"contribs.0.name":{"$exists":1}}}, 
{$project: {contribs:{$arrayElemAt:["$contribs",0]}}}
]
)

如何使用 Java 驱动程序 (2.14.1) 进行相同的查询？我尝试:

首先，我为 $match 阶段创建一个 DBObject:

DBObject match = new BasicDBObject("$match",new BasicDBObject("contribs.0.name",
                 new BasicDBObject("$exists",1)));

然后我创建一个BasicDBList:

BasicDBObject obj = new BasicDBObject("$contribs",0);

BasicDBList arrayElemAt = new BasicDBList();
arrayElemAt.add(obj);

这是$project阶段:

DBObject project1 = new BasicDBObject("$project", new BasicDBObject("contribs",
                    new BasicDBObject("$arrayElemAt",arrayElemAt)));

最后我创建聚合管道:

List<DBObject> list = new ArrayList<>();
list.add(match);
list.add(project1);

AggregationOutput output = this.coll.aggregate(list);

$Match 阶段有效，但 $project 无效。

我收到错误:“errmsg”:“无效的运算符 '$contribs'”，“code”:15999

Answer 1

当您打算创建List时，您创建了一个DBObject。另外，DBList 已被弃用一段时间了。习惯使用标准列表符号:

    List<DBObject> pipeline = Arrays.<DBObject>asList(
        new BasicDBObject(
            "$match",
            new BasicDBObject(
                "contribs.0", new BasicDBObject("$exists",true)
            )
        ),
        new BasicDBObject(
            "$project",
            new BasicDBObject(
                "contribs", new BasicDBObject("$arrayElemAt", Arrays.asList("$contribs",0))
            )
        )
    );

    AggregationOutput output = this.coll.aggrgate(pipeline);

另请注意，在现代驱动程序中，您确实应该使用 Document 代替所有 DBObject 类型。