db={"Users":[{id:"01",name:"john"},{id:"02",name:"donald"}],"Orders":[{id:"01",userId:"01"},{id:"02",userId:"02"},{id:"03",userId:"01"}]}

db.Users.aggregate([{$lookup:{from:"Orders",localField:"id",foreignField:"userId",as:"orders"}},{$match:{$expr:{$lt:[{$size:"$orders"},2]}}}])