Make Beatmap ISerializable and add more JsonIgnores

osu.Game/Beatmaps/BeatmapInfo.cs

@@ -15,6 +15,7 @@ namespace osu.Game.Beatmaps
     public class BeatmapInfo : IEquatable<BeatmapInfo>, IJsonSerializable, IHasPrimaryKey
     {
         [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+        [JsonIgnore]
         public int ID { get; set; }
 
         //TODO: should be in database
@@ -38,13 +39,16 @@ namespace osu.Game.Beatmaps
             set { onlineBeatmapSetID = value > 0 ? value : null; }
         }
 
+        [JsonIgnore]
         public int BeatmapSetInfoID { get; set; }
 
         [Required]
+        [JsonIgnore]
         public BeatmapSetInfo BeatmapSet { get; set; }
 
         public BeatmapMetadata Metadata { get; set; }
 
+        [JsonIgnore]
         public int BaseDifficultyID { get; set; }
 
         public BeatmapDifficulty BaseDifficulty { get; set; }
@@ -60,6 +64,7 @@ namespace osu.Game.Beatmaps
         [JsonProperty("file_sha2")]
         public string Hash { get; set; }
 
+        [JsonIgnore]
         public bool Hidden { get; set; }
 
         /// <summary>
@@ -74,6 +79,7 @@ namespace osu.Game.Beatmaps
         public float StackLeniency { get; set; }
         public bool SpecialStyle { get; set; }
 
+        [JsonIgnore]
         public int RulesetID { get; set; }
 
         public RulesetInfo Ruleset { get; set; }
@@ -116,6 +122,7 @@ namespace osu.Game.Beatmaps
         public string Version { get; set; }
 
         public double StarDifficulty { get; set; }
+        public bool ShouldSerializeStarDifficulty() => false;
 
         public bool Equals(BeatmapInfo other)
         {