How to lookup a field with an array in nested subdocument mongodb?
You can use lookup pipeline to handle nested lookup
const pipeline = [ { $match: {_id: new mongoose.Types.ObjectId(req.user.userId)} }, { $lookup: { from: 'levels', let: { level_id: "$teamInfo.challenges.levelId" }, pipeline: [ { $match: { $expr: { $eq: ["$_id", "$$level_id"] } } }, { $lookup: { from: '<level collection>', localField: "levelId", foreignField: "_id", as: "levelInfo" } }, { $project: { levelInfo: { name: "$levelInfo.name" } title: 1 } } ], as: "challenges" }, }, { $project: { _id: 1, fullname: 1, username: 1, teamInfo: { challenges: "$challenges" } }}]const result = await User.Aggregate(pipeline)hope this help !