Mongoose-在集合中未找到任何内容时,对其进行分组、计数并返回0

我的mongoDB数据库中有两个集合- users 和 connections . 每个 connection 有字段 user 其中包含 _id 一个 用户 储存在 用户 收藏。

我需要找到每个用户的连接总数,如果用户没有连接,它应该显示0作为他的总数。

还要查找连接数 date 字段不相等 '' 每个用户。

我写了这段代码,它会重新显示每个用户的连接总数,但只对至少有一个连接的用户,没有连接的用户根本不会出现。。。

Connection.aggregate(
                [
                    {
                        $group: {
                            _id: "$user",
                            total: { $sum: 1 },
                            withDate: { $sum: { $cond: { if: { $ne: ["$date", ""] }, then: 1, else: 0 } } }
                        }
                    },
                    {
                        $lookup: {
                            from: "users",
                            localField: "_id",
                            foreignField: "_id",
                            as: "users"
                        }
                    },
                    {
                        $unwind: "$users"
                    },
                    {
                        $project: {
                            Email: "$users.local.email",
                            "Last Edit": { $dateToString: { format: "%Y-%m-%d", date: "$users.local.lastConnectionEdit" } },
                            "Total Connections": "$total",
                            "Connections With Date": "$withDate"
                        }
                    }
                ],
                function(err, dashboardData) {
                    if (err) {
                        console.log(err);
                        res.status(500).end({ error: "Error fetching stats from database" });
                    } else {
                        let csv = json2csv(dashboardData);

                        res.attachment("stats.csv");
                        res.status(200).send(csv);
                    }
                }
            );

连接架构:

    active:Boolean,
    info: {
        name: String,
        profileImg: String,
        bio: String
    },
    connURL:String,
    handle:String,
    notes: String,
    date: String,
    value: {
        type: Number,
        min: 0,
        max: 5
    },
    tags: [String],
    user: {
        type: mongoose.Schema.Types.ObjectId,
        ref: 'User'
    }

用户架构:

local: {
        email: String,
        password: String,
        admin: Boolean,
        snoozeAmount: Number,
        snoozeInterval: String,
        emailIntervalWeeks: Number,
        emailIntervalDay: Number,
        isVerified: { type: Boolean, default: false },
        verifyToken: String,
        passwordResetToken: String,
        lastConnectionEdit: Date
    }