P3-播放列表应用 架构图 const Song = new Schema( { title: { type: String, required: true }, artist: { type: String, required: true }, releaseYear: { type: String, required: true }, genre: { type: String, required: true }, albumCover: { type: String, required: true }, album: {type: String, required: true}, songLength: { type: String, required: true }, // rating: {rating: Num