Mongoose - 查找每個用戶的最后一條消息

Question

我正在處理消息系統，我需要從向登錄用戶發送消息的每個用戶處獲取最后一條消息。 我在mongoDB中有這個結構：

[{
    "_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from每個向用戶發送消息的用戶的上一條消息中獲取字段， message和created字段。 我嘗試使用distinct但它只返回一個字段。 我有這個：

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)
        }
    });

但它返回向已登錄用戶發送消息的所有用戶，我需要只有唯一用戶和他們的最后一條消息。 有沒有辦法用貓鼬做到這一點？

Answer 1

使用聚合框架，其中您的管道階段具有$match ， $sort ， $group和$project表達式：

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" ，那么您的樣本集合的結果是：

{
    "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
}