猫鼬-查找每个用户的最后一条消息

问题描述

我正在使用消息系统，我需要从向已登录用户发送消息的每个用户那里获取最新消息.我在mongoDB中具有以下结构:

I'm working on message system and I need to get last message from each user who sent message to logged user. I have this structure in mongoDB:

[{
    "_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"
}]

我需要从向已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，但它仅返回一个字段.我有这个:

I need to get fields from, message and created from last message of each user who sent message to logged user. I tried use distinct but it returns only one field. I have this:

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)
        }
    });

，但是它将所有向已登录用户发送邮件的用户返回，我只需要唯一身份用户和他们的最后一条消息.有什么方法可以用猫鼬做这件事吗?

but it returns all users who sent messages to logged user and I need to have only unique users and their last message. Is there some way how to do it with mongoose?

推荐答案

在流水线阶段具有 $match ， $sort ， $group 和 $project 表达式:

Use aggregation framework where your pipeline stages have $match, $sort, $group and $project expressions:

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)
        }
    }
);

如果为req.user.username = "admin"，则与您的样本集合一起，结果为:

If req.user.username = "admin", with your sample collection then the result is:

{
    "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
}

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