Mongoose - 查找每个用户的最后一条消息

Question

我正在处理消息系统，我需要从每个向登录用户发送消息的用户那里获取最后一条消息。我在 mongoDB 中有这个结构：

[{
    "_id": "551bd621bb5895e4109bc3ce",
    "from": "admin",
    "to": "user1",
    "message": "message1",
    "created": "2015-04-01T11:27:29.671Z"
}, {
    "_id": "551bd9acf26208ac1d9b831d",
    "from": "user1",
    "to": "admin",
    "message": "message2",
    "created": "2015-04-01T11:42:36.936Z"
}, {
    "_id": "551bdd6d849d53001dd8a64a",
    "from": "user1",
    "to": "user2",
    "message": "message3",
    "created": "2015-04-01T11:58:37.858Z"
}, {
    "_id": "551bdd99849d53001dd8a64b",
    "from": "user2",
    "to": "admin",
    "__v": 0,
    "message": "message4",
    "created": "2015-04-01T11:59:21.005Z"
}, {
    "_id": "551bdda1849d53001dd8a64c",
    "from": "user1",
    "to": "admin",
    "__v": 0,
    "message": "message5",
    "created": "2015-04-01T11:59:29.971Z"
}]

我需要从向登录用户发送消息的每个用户的最后一条消息中获取字段 from、message 和 created。我尝试使用 distinct，但它只返回一个字段。我有这个：

Message.find({
        to: req.user.username
    })
    .select('message created')
    .sort('-created')
    .exec(function (err, messages) {
        if (err) {
            return res.status(400).send({
                message: getErrorMessage(err)
            });
        } else {
            res.json(messages)
        }
    });

但它会返回所有向登录用户发送消息的用户，我只需要拥有唯一用户和他们的最后一条消息。有没有办法用猫鼬做到这一点？

Answer 1

在您的管道阶段具有 $match、$sort、$group 和 $project 表达式的情况下使用聚合框架：

Message.aggregate(
    [
        // Matching pipeline, similar to find
        { 
            "$match": { 
                "to": req.user.username
            }
        },
        // Sorting pipeline
        { 
            "$sort": { 
                "created": -1 
            } 
        },
        // Grouping pipeline
        {
            "$group": {
                "_id": "$from",
                "message": {
                    "$first": "$message" 
                },
                "created": {
                    "$first": "$created" 
                }
            }
        },
        // Project pipeline, similar to select
        {
             "$project": { 
                "_id": 0,
                "from": "$_id",
                "message": 1,
                "created": 1
            }
        }
    ],
    function(err, messages) {
       // Result is an array of documents
       if (err) {
            return res.status(400).send({
                message: getErrorMessage(err)
            });
        } else {
            res.json(messages)
        }
    }
);

如果req.user.username = "admin"，使用您的样本集合，那么结果是：

{
    "result" : [ 
        {
            "message" : "message4",
            "created" : "2015-04-01T11:59:21.005Z",
            "from" : "user2"
        }, 
        {
            "message" : "message5",
            "created" : "2015-04-01T11:59:29.971Z",
            "from" : "user1"
        }
    ],
    "ok" : 1
}