I want to add join collections using $lookup in mongodb. I am trying as below

{
 $lookup:{
   from:"User",
   localField:"assignedId",
   foreignField:"_id",
   as:"dataa"}
}

Now I have two collections

User contains objectid of users like "_id" : ObjectId("56ab6663d69d2d1100c074db"),

and Tasks where it contains assignedId as a string "assignedId":"56ab6663d69d2d1100c074db"

Now, when applying $lookup in both collection its not working because Id's are not matching.

For that I googled it and found a solution that to include

{ $project: { assignedId: {$toObjectId: "$assignedId"} }}

but this solution is not working for me, Its throwing an error:

assert: command failed: { "ok" : 0, "errmsg" : "invalid operator '$toObjectId'", "code" : 15999 } : aggregate failed

Please help me how can I resolve this issue.

Thanks