Node.js 条件内的表达式

这是一个跟进问题:

我只想为类型为‘Buy’的文档汇总集合的平均值. 因此，问题是如何获得sum表达式周围的if条件以及totalBuyAmount的相同条件.判断代码中的注释:)

以下是示例文档:

[
  {
    _id: ObjectId("653c4c017800342a3bedb82e"),
    type: 'buy',
    item: 555,
    amount: 1,
    priceEach: 100000
  },
  {
    _id: ObjectId("653c4c017800342a3bedb830"),
    type: 'buy',
    item: 384,
    amount: 2,
    priceEach: 70000
  },
  {
    _id: ObjectId("653c4c017800342a3bedb831"),
    type: 'buy',
    item: 384,
    amount: 1,
    priceEach: 50000
  },
  {
    _id: ObjectId("653c4c017800342a3bedb831"),
    type: 'sell',
    item: 384,
    amount: -1,
    priceEach: 50000
  }
]

仅当type=‘Buy’时，我才需要totalBuyPrice

db.collection.aggregate([
  {
    $group: {
      _id: "$item",
      totalBuyPrice: {
        $sum: { // Only if type === 'buy'
          $multiply: [
            "$priceEach",
            "$amount"
          ]
        }
      },
      totalBuyAmount: // $sum $amount if type === 'buy'
      totalAmount: {
        $sum: "$amount"
      }
    }
  },
  {
    $project: {
      _id: 1,
      avg: {
        $divide: [
          "totalBuyPrice",
          "totalBuyAmount"
        ]
      },
      amount: "$totalAmount"
    }
  }
])