[{"_id":ObjectId("6099614e503dd75c29c9715d"),"systemDelete_DT":null,"group":ObjectId("6099614e503dd75c29c9715b"),"headers":[{"_id":ObjectId("609e875203041759d7b0428a"),"systemDelete_DT":"2021-05-14T15:21:06+01:00","fieldName":"Field1-X"},{"_id":ObjectId("609fe89e5d4ac02438abdc31"),"systemDelete_DT":null,"fieldName":"Field1"},{"_id":ObjectId("60a00c14543d582c151e34f6"),"systemDelete_DT":null,"fieldName":"Field2"}],"list":[{"_id":ObjectId("609966bccce7575f2408fddc"),"systemDelete_DT":null,"text":"abc1","customData":[{"_id":ObjectId("609e875203041759d7b0428b"),"systemDelete_DT":"2021-05-15T19:39:20+01:00","field_id":ObjectId("609e875203041759d7b0428a"),"fieldText":"Example-X"},{"_id":ObjectId("609fe89e5d4ac02438abdc32"),"systemDelete_DT":null,"field_id":ObjectId("609fe89e5d4ac02438abdc31"),"fieldText":"Example1"},{"_id":ObjectId("60a00c14543d582c151e34f7"),"systemDelete_DT":null,"field_id":ObjectId("60a00c14543d582c151e34f6"),"fieldText":"Example2"}]},{"_id":ObjectId("609966becce7575f2408fdde"),"systemDelete_DT":null,"text":"abc2","customData":[{"_id":ObjectId("609e875203041759d7b0428b"),"systemDelete_DT":"2021-05-15T19:39:20+01:00","field_id":ObjectId("609e875203041759d7b0428a"),"fieldText":"Example-X"},{"_id":ObjectId("609fe89e5d4ac02438abdc32"),"systemDelete_DT":null,"field_id":ObjectId("609fe89e5d4ac02438abdc31"),"fieldText":"Example3"},{"_id":ObjectId("60a00c14543d582c151e34f7"),"systemDelete_DT":null,"field_id":ObjectId("60a00c14543d582c151e34f6"),"fieldText":"Example4"}]},{"_id":ObjectId("609966d2474ad05f369c891a"),"systemDelete_DT":"2021-05-14T15:21:06+01:00","text":"abc32","customData":[{"_id":ObjectId("609e875203041759d7b0428b"),"systemDelete_DT":"2021-05-15T19:39:20+01:00","field_id":ObjectId("609e875203041759d7b0428a"),"fieldText":""},{"_id":ObjectId("609fe89e5d4ac02438abdc32"),"systemDelete_DT":null,"field_id":ObjectId("609fe89e5d4ac02438abdc31"),"fieldText":"Example5"},{"_id":ObjectId("60a00c14543d582c151e34f7"),"systemDelete_DT":null,"field_id":ObjectId("60a00c14543d582c151e34f6"),"fieldText":"Example6"}]},]}]

db.collection.aggregate([{$unwind:"$list"},{$addFields:{"list.customData":{$filter:{input:"$list.customData",cond:{$eq:["$$this.systemDelete_DT",null]}}}}},{$addFields:{"list.customData":{$map:{input:"$list.customData",as:"cData",in:{"$mergeObjects":["$$cData",{$arrayElemAt:[{$map:{input:{$filter:{input:"$headers",as:"head",cond:{$eq:["$$cData.field_id","$$head._id"]}}},in:{fieldName:"$$this.fieldName"}}},0]}]}}}}},{"$replaceRoot":{"newRoot":"$list"}},{$unwind:"$customData"},{"$replaceRoot":{"newRoot":"$customData"}}])