db.Tasks.aggregate([ {
$lookup:

{ from: "Userdata", localField: "customer_id", foreignField: "_id", as: "userdetails" }

} ]

here i get the empty array of userdetails .
please check i gave the response below.
response:
[

{ "_id": "5a6ecde19f3a0d31f4cd5b5e", "executive_id": "5a6ec6879a3e3406f8f7d70b", "customer_id": "5a5495b1a03790158019a7d5", "task_completed": false, "__v": 0, "userdeatails": [] }

,

{ "_id": "5a6ecdf29f3a0d31f4cd5b5f", "executive_id": "5a6ec6879a3e3406f8f7d70b", "customer_id": "5a5495b1a03790158019a7d5", "task_completed": false, "__v": 0, "userdeatails": [] }

,

{ "_id": "5a6ece0d9f3a0d31f4cd5b60", "executive_id": "5a6ec6879a3e3406f8f7d70b", "customer_id": "5a67055d611e836db9078ca3", "task_completed": false, "__v": 0, "userdeatails": [] }

,

{ "_id": "5a6ed8829f3a0d31f4cd5b61", "executive_id": "5a6ec822c2eee725f8282b3c", "customer_id": "5a67055d611e836db9078ca3", "task_completed": false, "__v": 0, "userdeatails": [] }

]