我使用MongoDB和Mongoose作为我的ODM,我正在尝试使用populate和group by在同一语句中进行查询.

这是我的简单文档模型：

var userSchema = new Schema({
    username: String
});

var messageSchema = new Schema({
    from: { type: Schema.ObjectId, ref: 'User' },
    to: { type: Schema.ObjectId, ref: 'User' },
    message: String,
    date: { type: Date, default: Date.now }
});

我只是试图为每个用户收集每条消息,由他与之交谈的每个用户进行分组.我试过这样的：

this.find({ 'to': user })
    .sort({ 'date': 1 })
    .group('from')
    .populate(['from', 'to'])
    .exec(callback);

但是,不幸的是,我的模型没有组方法.你有任何解决方案,让这个工作吗？

谢谢.

解决方法:

使用$lookup populate的示例,lookup填充为数组,因此$unwind.

Message.aggregate(
    [
        { "$match": { "to": user } },
        { "$sort": { "date": 1 } },
        { "$group": { 
            "_id": "from",
            "to": { "$first": "$to" },
            "message": { "$first": "$message" },
            "date": { "$first": "$date" },
            "origId": { "$first": "$_id" }
        }},
        { "$lookup": {
             "from": "users",
             "localField": "from",
             "foreignField": "_id",
             "as": "from"
        }},
        { "$lookup": {
             "from": "users",
             "localField": "to",
             "foreignField": "_id",
             "as": "to"
        }},
        { "$unwind": { "path" : "$from" } },
        { "$unwind": { "path" : "$to" } }
    ],
    function(err,results) {
        if (err) throw err;
        return results;
    }
)