Mongodb 3.2 : mongodb aggregation with $project before $lookup

Question

I've a problem, actually I'm trying to join different collections for some operation, but I'm not able to get the result as both "BatchInfo" and "UserInfo" is coming as null.

The reason I'm doing "project" first is because I want to take out "Batch Id" and "User Id" from respective objects and then apply in respective "lookup" "foreignField". Also, I'm using mongo 3.2.

db.getCollection('coursecompletedfeedbacks').aggregate([

    {

        $project: {

           "BATCH" : 1,

           "FEEDBACK" : 1,

           "USER" : 1,

           "batchId" : "$BATCH._id",

           "userId" : "$USER._id"

        }

    },

    {

        $lookup:{

            from: "batches",           

            localField: "batchId",         

            foreignField: "_id",   

            as: "BatchInfo"           

        }

    },

    {

        $lookup:{

            from: "users",           

            localField: "userId",         

            foreignField: "_id",   

            as: "UserInfo"           

        }

    }

])

Answer 1

Try as below:

db.getCollection('coursecompletedfeedbacks').aggregate([
    {
        $lookup:{
            from: "batches",           
            let: { bId: "$BATCH._id" },          
            pipeline: [
                {
                    $match: {
                        $expr: {
                            $eq: ["$_id", "$$bId"]
                        }
                    }
                },
            ],
            as: "BatchInfo"           
        }
    },
    {
        $lookup:{
            from: "users",           
            let: { uId: "$USER._id" },      
            pipeline: [
                {
                    $match: {
                        $expr: {
                            $eq: ["$_id", "$$uId"]
                        }
                    }
                },
            ],   
            as: "UserInfo"           
        }
    },
    {
        $project: {
           "BatchInfo" : 1,
           "UserInfo" : 1,
           ""FEEDBACK" : 1,
        }
    },
])