我在MongoDB Aggregation阶段有以下文档：

_id: ObjectId('641316fd596514e63a2121ca'),
teamId: 5,
playerId: ObjectId('64131139596514e63a2121c1')

_id: ObjectId('641316fd596514e63a2121ca'),
teamId: 5,
playerId: ObjectId('64143f7702ede13fea5ff3be')

_id: ObjectId('64143f458b6a6cf54652d3a5'),
teamId: 17,
playerId: ObjectId('64143fa75a97b9a63a260e16')

_id: ObjectId('64143f4f9ba5f2b3cf3030e6'),
teamId: 3

我想创建一个名为players的Object字段，这个对象应该包含playerId和gender: 'male'，但前提是文档已经有了playerId属性。如果没有，它应该什么都不做。所以结果应该是这样的：

_id: ObjectId('641316fd596514e63a2121ca'),
teamId: 5,
players: {
    gender: 'male',
    playerId: ObjectId('64131139596514e63a2121c1')
}

_id: ObjectId('641316fd596514e63a2121ca'),
teamId: 5,
players: {
    gender: 'male',
    playerId: ObjectId('64143f7702ede13fea5ff3be')
}

_id: ObjectId('64143f458b6a6cf54652d3a5'),
teamId: 17,
players: {
    gender: 'male',
    playerId: ObjectId('64143fa75a97b9a63a260e16')
}

_id: ObjectId('64143f4f9ba5f2b3cf3030e6'),
teamId: 3

我试着这样做：

db.collection.aggregate([
  {
    $set: {
      players: {
        $cond: {
          if: {
            $ne: [
              "$players",
              undefined
            ]
          },
          then: {
            $mergeObjects: [
              "$players",
              {
                "gender": "male"
              }
            ]
          },
          else: "$$REMOVE"
        }
      }
    }
  }
])

但即使文档中没有playerId属性，这也会添加一个对象。