db={"products":[{"id":1,"name":"product 1"},],"features":[{"id":101,"name":"width"},{"id":102,"name":"length"},{"id":103,"name":"height"}],"productfeatures":[{"productId":1,"featureId":101,"value":"3"},{"productId":1,"featureId":102,"value":"4"},{"productId":1,"featureId":103,"value":"5"}]}

db.products.aggregate([{$match:{id:1}},{$lookup:{from:"productfeatures",localField:"id",foreignField:"productId",as:"productfeatures"}}])