db={"users":[{"_id":"60499e72b60a8819c4e0fa03","LastName":"Doe","FirstName":"John"}],"userdocuments":[{"_id":"61025b9f890bacbe8f450f6a","userid":"60499e72b60a8819c4e0fa03",documents:{documentOne:{documentTitle:"This is Document One"},documentTwo:{documentTitle:"This is Document Two"},documentThree:{documentTitle:"This is Document Three"}}}]}

db.users.aggregate([{$lookup:{from:"userdocuments",localField:"_id",foreignField:"userid",as:"Documents"}},{$addFields:{Documents:{$arrayElemAt:["$Documents.documents",0]}}}])