我有3个具有多对多关系的mongodb集合，productfeatures集合是关系集合。db={"products":[{"id":1,"name":"product1"}],"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"},{"produ