如何根据特定条件在mongodb聚合中进行$sum或$subtract

假设我有一个这样的模型

userId: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'User',
    required: true,
},
points: {
    type:Number,
    required: true,
},
wonOrLost:{
    type: String,
    required: true,
    enum: ['won', 'lost', 'noResults'],
    default: 'noResults',
},

然后我得到了如下的聚合代码：

let _points = await Points.aggregate([
    { $match: {
        userId: user._id ,
        wonOrLost: { $ne: "noResults" }
    }},
    { $group: {
        _id: "$userId",
        // here(totalPoints) I want to add points if wonOrLost == 'won' and subtract if wonOrLost == 'lost'
        totalPoints: { $sum: "$points" }, 
    }}    
])

---

下面是我使用的方法，如果在求和之前匹配失败，策略是将

点

乘以

-1

let _points = await Points.aggregate([
    {
        $match: {
            userId: user._id,
            wonOrLost: {$ne: 'noResults'}
        }
    },
    {
        $group: {
            _id: '$userId',
            totalPoints: {$sum: {$cond: [{$eq: ["$wonOrLost", "won"]}, '$points', {$multiply: ['$points', -1]}]}},
        }
    }
]);