[{"_id":"1","name":"document1","parentId":null},{"_id":"2","name":"folder1","parentId":null},{"_id":"3","name":"folder2","parentId":"2"},{"_id":"4","name":"document2","parentId":"2"},{"_id":"5","name":"document3","parentId":"3"}]

db.collection.aggregate([{"$match":{name:{$regex:"folder"}}},{"$lookup":{"from":"collection","localField":"_id","foreignField":"parentId","as":"contentIds"}},{"$addFields":{"contentIds":{"$map":{"input":"$contentIds","in":"$$this._id"}}}}])