猫鼬聚合的问题

Question

我的要求是让特定用户获得根据月分组的所有费用，并返回月和该月的总费用。 我有如下收藏。

expenses = {{
        "_id": {
            "$oid": "5bc0f74df46dae0bf4ffdc39"
        },
        "user": {
            "$oid": "5bab847a5b0d2e2ce8b4cbe5"
        },
        "date": "15-Mar-2018",
        "expenseInfo": "Clothes",
        "category": "Shopping",
        "amount": 100,
        "__v": 0
    },
    {
        "_id": {
            "$oid": "5bc0f78bf46dae0bf4ffdc3b"
        },
        "user": {
            "$oid": "5bab847a5b0d2e2ce8b4cbe5"
        },
        "date": "11-Apr-2018",
        "expenseInfo": "mobile",
        "category": "Bills",
        "amount": 100,
        "__v": 0
    },
    {
        "_id": {
            "$oid": "5bc0f76cf46dae0bf4ffdc3a"
        },
        "user": {
            "$oid": "5bab847a5b0d2e2ce8b4cbe5"
        },
        "date": "18-Apr-2018",
        "expenseInfo": "Petrol",
        "category": "Fuel",
        "amount": 20,
        "__v": 0
    }
    }

这是我的费用模式。 另外，请注意，用户对象（用户是费用模式中引用的其他模式）

 const ExpenseSchema = new mongoose.Schema({
        user: {
            type: mongoose.Schema.Types.ObjectId,
            ref: 'user'
        },
        date: {
            type: String
        },
        expenseInfo: {
            type: String,
            required: true
        },
        category: {
            type: String,
            required: true
        },
        amount: {
            type: Number,
            required: true
        }
    });

我尝试过类似的操作，但是没有任何数据。这是什么问题？

Expense.aggregate([{
                $match: {
                    "_id": request.user.id
                }
            },

            {
                "$group": {
                    "_id": {
                        "$arrayElemAt": [{
                            "$split": ["$date", "-"]
                        }, 1]
                    },
                    "Total": {
                        "$sum": "$amount"
                    }
                }
            }
        ])