mongodb $lookup for nested object in array with projection

Question

I'm having a problem using $lookup in my aggregation pipeline.

I have 2 collections, members & messages

members :

{_id, FirstName, LastName, Email, ...}

messages

{
  _id:ObjectId('xxx'),
  createdBy:ObjectId(''),
  ...
  threads:[
    {  message:'' , attachments:[] , from:ObjectId , to:[{status:'Read' , recipient:ObjectId}] }]
}

What I'm trying to do is,

lookup for each recipient in : to:[{status:'Read' , recipient:ObjectId}] and populate name and email from members collection.

I tried many different things like this one; //

db.messages.aggregate([
     {
                '$lookup': {
                    'from': 'members',
                    'let': {
                        'memberId': '$threads.to.recipient'
                    },
                    'pipeline': [
                        {
                            '$match': {
                                '$expr': {
                                    '$eq': [
                                        '$$memberId', '$members._id'
                                    ]
                                }
                            }
                        },
                        {$project: {FirstName: 1, _id: 1, LastName: 1, Email: 1}}
                    ],
                    'as': 'members'
                }
            }
    ]

Many different queries including this one always return [] for members ('as': 'members').

Just to test I tired with mongoose and .populate('threads.to.recipient','FirstName') worked perfectly. But I cannot use mongoose for this I have to use MongoDB's native nodejs driver.

any advice would be greatly appreciated on this...

Tiya Jose Tiya Jose · Accepted Answer · 2020-10-14T10:07:22

You have to use $unwind to flatten the structure of threads array before performing $lookup

db.messages.aggregate([
  {
    $unwind: "$threads"
  },
  {
    $unwind: "$threads.to"
  },
  {
    $lookup: {
      from: "members",
      let: {
        memberId: "$threads.to.recipient"
      },
      as: "members",
      pipeline: [
        {
          $match: {
            $expr: {
              $eq: [
                "$$memberId",
                "$_id"
              ]
            }
          }
        },
        {
          $project: {
            FirstName: 1,
            _id: 1,
            LastName: 1,
            Email: 1
          }
        }
      ]
    }
  }
])

See the working example in MongoDB Playground

If you don't want to use $unwind, just try the below query:

db.messages.aggregate([
  {
    "$lookup": {
      "from": "members",
      "localField": "threads.to.recipient",
      "foreignField": "_id",
      "as": "members"
    }
  }
])

See the working example in MongoDB Playground

mongodb $lookup for nested object in array with projection

1 Answers