MongoDB aggregate: $lookup by _id, then get first element value
You can achieve that by appending a $project
stage using the $arrayElemAt operator to your aggregation pipeline like this
// your previous stages incl. lookup...$project: { fromUser: { $arrayElemAt: ["$fromUser.userName", 0] }, toUser: { $arrayElemAt: ["$toUser.userName", 0] }}...