使用对象数组进行MongoDB查找

Question

我有三个收集如下： collectionA：

_id: ObjectID('ID HERE'),
......
......
......
company: {
  id: ObjectId('id of company'),
  name: "Name of company"
},
conductors: [
  {
     name: "User1",
     userId: ObjectId("id of user 1"),
     _id: ObjectId('Id')
  },
  {
    name: "New User",  // This user is not existed in system, so only his/her name exists.
    _id: ObjectId('id')
  },
  {
     name: "User2",
     userId: ObjectId("id of user 2"),
     _id: ObjectId('Id')
  },
]

company收集是：

{
  _id: ObjectId('company id'),
  name: "company name",
  ......
  ......
}

和另一个集合user：

{
  _id: ObjectId('id of user'),
  firstName: "User1",
  profilePic: {
    name: "",
    size: ""
    container: "",
    .......
    .......
  }
}

我想要的是从collectionA获取，并使用集合user查找以获取用户的其他详细信息，例如profilePic，但是对于用户表中不存在的新用户，以及字段userId为空或不存在，我应该选择空profilePic，我已经尝试使用$unwind，但它只返回一个指挥（一个数组的elmenet）。

db.getCollection('collectionA').aggregate([
    {"$match":{
        "expireDate":{"$gt": ISODate("2018-03-31T04:35:43.693Z")},
        "publishDate":{"$lt": ISODate("2018-04-02T00:00:00.000Z")},
        "status":1
    }},
    {"$lookup":{
        "from":"companies",
        "localField":"company.id",
        "foreignField":"_id",
        "as":"companyDetails"
    }},
    {"$match":{"companyDetails":{"$ne":[]}}},
//     {$unwind: "$conductors"},
//     {
//         $lookup:{
//             from: "users", 
//             localField: "conductors.userId", 
//             foreignField: "_id",
//             as: "conductorUsers"
//         }
//     },
    // Unwind the result arrays ( likely one or none )
//     { "$unwind": "$conductorUsers" },
    // Group back to arrays
//     { "$group": {
//         "_id": "$_id",
//         "conductors": { "$push": "$conductors" },
//         "conductorUsers": { "$push": "$conductorUsers" }
//     }},
    {"$project":{
//         "conductors": { "$push": "$conductors" },
//         "conductorUsers": { "$push": "$conductorUsers" },
//         "conductorUsers.firstName": 1,
//         "conductorUsers.lastName": 1,
//         "conductorUsers.profilePic": { $ifNull: [ "$conductorUsers.profilePic", "" ] },
//         "conductorUsers._id": 1,
        "title":1,
        "description":1,
        "startDate":1,"endDate":1,"publishDate":1,"expireDate":1,"type":1,
        "location":1,"price":1,"contactNumbers":1,"emails":1,"link":1,
        "conductors":1,"attachment":1,"company":1,"views":1,
        "status":{"$cond":{
            "if":{"$and":[
                {"$lt":["$publishDate", ISODate("2018-04-02T00:00:00.000Z")]},
                {"$gt":["$publishDate", ISODate("2018-03-31T00:00:00.000Z")]}
            ]},
            "then":"New",
            "else":{"$cond":{
                "if":{
                    "$lt":["$publishDate", ISODate("2018-03-31T00:00:00.000Z")]
                },"then":"Old","else":"Future"
            }}
        }},
        "companyDetails.profilePic":1,"companyDetails.totalRatingAverage":1,
        "companyDetails.name":1,"companyDetails.description":1,"companyDetails.website":1,
        "companyDetails.founded":1,"companyDetails.benefits":1,
        "expiringDuration":{"$floor":{
            "$divide":[{"$subtract":["$expireDate", ISODate("2018-03-31T00:00:00.000Z")]},86400000]
        }},
        "companyDetails.totalReviews":{"$size":{"$ifNull":[
            {"$let":{
                "vars":{
                  "companyDetailOne":{"$arrayElemAt":["$companyDetails",0]}
                },"in":"$$companyDetailOne.reviews"
            }},[]
        ]}}
     }} 
])