Perform $lookup with object value in array from pipeline
You can put $lookup stage inside pipeline,
$lookupwith users collection$addFields,$arrayElemAtto get first element from above user lookup result
Property.aggregate([ { $match: { _id: ObjectId(propertyId) } }, { $lookup: { from: "propertytestimonials", let: { propPropertyId: "$_id" }, pipeline: [ { $match: { $expr: { $eq: ["$propertyId", "$$propPropertyId"] } } }, { $lookup: { from: "users", localField: "userId", foreignField: "_id", as: "user" } }, { $addFields: { user: { $arrayElemAt: ["$user", 0] } } } ], as: "testimonials" } }])