Database
db={"users":[{"_id":ObjectId("6471bf9db77f0b33a9b0bd38"),"name":"Alok","city":"someCity"}],"order":[{"_id":ObjectId("6471bfd4b77f0b33a9b0bd39"),"uid":"6471bf9db77f0b33a9b0bd38","product":"product1"}]}
Query
db.order.aggregate([{$set:{uid:{$toObjectId:"$uid"}}},{$lookup:{from:"users",localField:"uid",foreignField:"_id",as:"join"}}])