我想知道如何使用$lookup和pipeline将**localField添加到lookup field中。
因此，我有一个user模式，如下所示（我希望在最终结果中保留"added_at"**字段）

User {
   _id: ObjectId("123456789xx")
   first_name: "Sam",
   last_name: "Jones",
   email: "samjones@gmail.com",
   inbox: [
      {
          participant: ObjectId("1XXXXXXXXX"),
          added_at: "12:00:00 09/21/2021"
      },
      {
          participant: ObjectId("2XXXXXXXXX"),
          added_at: "12:00:00 11/21/2022"
      },
   ]
}

现在我使用$lookup填充**participant**字段

{
   from: "Users",
   localField: "inbox.participant",
   foreignField:"_id",
   as: "participants",
   pipeline:[
      { $project: {first_name:1, last_name:1, _id: 1}}
   ]
}

然后我得到：

{
  _id: ObjectId("123456789xx")
  first_name: "Sam",
  last_name: "Jones",
  email: "samjones@gmail.com",
  inbox:...,
  participants: [
     { _id: ObjectId("1xxxxx"), first_name: "John", last_name:"Doe"},
     { _id: ObjectId("2xxxxx"), first_name: "Jack", last_name: "Smith"}
  ]
}

我希望能够在每个参与者中保留**"added_at"**字段，如何才能做到这一点？
谢谢