db={"orderTracking":[{_id:ObjectId("59fb7815b3b8429f4750b0df"),itemName:"Hamam Soap",userId:12,TrackLocation:[{locationId:1,at:"2017-10-11"},{locationId:2,at:"2017-10-13"}],userId:12,price:20}],"locationType":[{_id:ObjectId("59b2111345cb72345a35fefd"),locationId:1,productTypeName:"Warehouse"},{_id:ObjectId("59af8ce445cb72345a35feea"),locationId:2,productTypeName:"On Transit"}],User:[{_id:ObjectId("59a504eb6171b554c02292a9"),"userName":"Shahabaz Shafi","dateOfBirth":"1992-01-01",userId:12,"addres":{"country":"India","state":"Karnataka","city":"Bengaluru"}}]}

db.User.aggregate([{"$lookup":{"from":"orderTracking","let":{"userId":"$userId"},"pipeline":[{"$match":{"$expr":{"$eq":["$userId","$$userId"]}}},{$unwind:"$TrackLocation"},{"$lookup":{"from":"locationType","let":{"location":"$TrackLocation.locationId"},"pipeline":[{"$match":{"$expr":{"$eq":["$locationId","$$location"]}}}],"as":"locationType"}},{$project:{productTypeName:{$arrayElemAt:["$locationType.productTypeName",0]},at:"$TrackLocation.at"}}],"as":"locationType"}},{"$replaceRoot":{"newRoot":{"$mergeObjects":["$addres","$$ROOT"]}}},{"$project":{"addres":0}}])