diff --git a/osu.Game/Beatmaps/BeatmapInfo.cs b/osu.Game/Beatmaps/BeatmapInfo.cs
index ac5b5d7a8a..3bcc00f5de 100644
--- a/osu.Game/Beatmaps/BeatmapInfo.cs
+++ b/osu.Game/Beatmaps/BeatmapInfo.cs
@@ -178,7 +178,7 @@ namespace osu.Game.Beatmaps
 
         #region Implementation of IHasOnlineID
 
-        public int? OnlineID => OnlineBeatmapID;
+        public int OnlineID => OnlineBeatmapID ?? -1;
 
         #endregion
 
diff --git a/osu.Game/Beatmaps/BeatmapSetInfo.cs b/osu.Game/Beatmaps/BeatmapSetInfo.cs
index 8b01831b3c..e8c77e792f 100644
--- a/osu.Game/Beatmaps/BeatmapSetInfo.cs
+++ b/osu.Game/Beatmaps/BeatmapSetInfo.cs
@@ -91,7 +91,7 @@ namespace osu.Game.Beatmaps
 
         #region Implementation of IHasOnlineID
 
-        public int? OnlineID => OnlineBeatmapSetID;
+        public int OnlineID => OnlineBeatmapSetID ?? -1;
 
         #endregion
 
diff --git a/osu.Game/Database/IHasOnlineID.cs b/osu.Game/Database/IHasOnlineID.cs
index c55c461d2d..529c68a8f8 100644
--- a/osu.Game/Database/IHasOnlineID.cs
+++ b/osu.Game/Database/IHasOnlineID.cs
@@ -8,8 +8,8 @@ namespace osu.Game.Database
     public interface IHasOnlineID
     {
         /// <summary>
-        /// The server-side ID representing this instance, if one exists.
+        /// The server-side ID representing this instance, if one exists. -1 denotes a missing ID.
         /// </summary>
-        int? OnlineID { get; }
+        int OnlineID { get; }
     }
 }
diff --git a/osu.Game/Database/RealmContextFactory.cs b/osu.Game/Database/RealmContextFactory.cs
index 82d51e365e..078709ad2d 100644
--- a/osu.Game/Database/RealmContextFactory.cs
+++ b/osu.Game/Database/RealmContextFactory.cs
@@ -2,12 +2,14 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Linq;
 using System.Threading;
 using osu.Framework.Allocation;
 using osu.Framework.Development;
 using osu.Framework.Logging;
 using osu.Framework.Platform;
 using osu.Framework.Statistics;
+using osu.Game.Models;
 using Realms;
 
 #nullable enable
@@ -26,7 +28,12 @@ namespace osu.Game.Database
         /// </summary>
         public readonly string Filename;
 
-        private const int schema_version = 6;
+        /// <summary>
+        /// Version history:
+        /// 6  First tracked version (~20211018)
+        /// 7  Changed OnlineID fields to non-nullable to add indexing support (20211018)
+        /// </summary>
+        private const int schema_version = 7;
 
         /// <summary>
         /// Lock object which is held during <see cref="BlockAllOperations"/> sections, blocking context creation during blocking periods.
@@ -120,6 +127,31 @@ namespace osu.Game.Database
 
         private void onMigration(Migration migration, ulong lastSchemaVersion)
         {
+            if (lastSchemaVersion < 7)
+            {
+                convertOnlineIDs<RealmBeatmap>();
+                convertOnlineIDs<RealmBeatmapSet>();
+                convertOnlineIDs<RealmRuleset>();
+
+                void convertOnlineIDs<T>() where T : RealmObject
+                {
+                    var className = typeof(T).Name.Replace(@"Realm", string.Empty);
+
+                    var oldItems = migration.OldRealm.DynamicApi.All(className);
+                    var newItems = migration.NewRealm.DynamicApi.All(className);
+
+                    int itemCount = newItems.Count();
+
+                    for (int i = 0; i < itemCount; i++)
+                    {
+                        var oldItem = oldItems.ElementAt(i);
+                        var newItem = newItems.ElementAt(i);
+
+                        long? nullableOnlineID = oldItem?.OnlineID;
+                        newItem.OnlineID = (int)(nullableOnlineID ?? -1);
+                    }
+                }
+            }
         }
 
         /// <summary>
diff --git a/osu.Game/Models/RealmBeatmap.cs b/osu.Game/Models/RealmBeatmap.cs
index 5049c1384d..9311425cb7 100644
--- a/osu.Game/Models/RealmBeatmap.cs
+++ b/osu.Game/Models/RealmBeatmap.cs
@@ -44,7 +44,8 @@ namespace osu.Game.Models
         [MapTo(nameof(Status))]
         public int StatusInt { get; set; }
 
-        public int? OnlineID { get; set; }
+        [Indexed]
+        public int OnlineID { get; set; } = -1;
 
         public double Length { get; set; }
 
diff --git a/osu.Game/Models/RealmBeatmapSet.cs b/osu.Game/Models/RealmBeatmapSet.cs
index 314ca4494b..d6e56fd61c 100644
--- a/osu.Game/Models/RealmBeatmapSet.cs
+++ b/osu.Game/Models/RealmBeatmapSet.cs
@@ -20,7 +20,8 @@ namespace osu.Game.Models
         [PrimaryKey]
         public Guid ID { get; set; } = Guid.NewGuid();
 
-        public int? OnlineID { get; set; }
+        [Indexed]
+        public int OnlineID { get; set; } = -1;
 
         public DateTimeOffset DateAdded { get; set; }
 
@@ -62,7 +63,7 @@ namespace osu.Game.Models
             if (IsManaged && other.IsManaged)
                 return ID == other.ID;
 
-            if (OnlineID.HasValue && other.OnlineID.HasValue)
+            if (OnlineID >= 0 && other.OnlineID >= 0)
                 return OnlineID == other.OnlineID;
 
             if (!string.IsNullOrEmpty(Hash) && !string.IsNullOrEmpty(other.Hash))
diff --git a/osu.Game/Models/RealmRuleset.cs b/osu.Game/Models/RealmRuleset.cs
index 0dcd701ed2..5d70324713 100644
--- a/osu.Game/Models/RealmRuleset.cs
+++ b/osu.Game/Models/RealmRuleset.cs
@@ -18,7 +18,8 @@ namespace osu.Game.Models
         [PrimaryKey]
         public string ShortName { get; set; } = string.Empty;
 
-        public int? OnlineID { get; set; }
+        [Indexed]
+        public int OnlineID { get; set; } = -1;
 
         public string Name { get; set; } = string.Empty;
 
@@ -29,7 +30,7 @@ namespace osu.Game.Models
             ShortName = shortName;
             Name = name;
             InstantiationInfo = instantiationInfo;
-            OnlineID = onlineID;
+            OnlineID = onlineID ?? -1;
         }
 
         [UsedImplicitly]
@@ -39,7 +40,7 @@ namespace osu.Game.Models
 
         public RealmRuleset(int? onlineID, string name, string shortName, bool available)
         {
-            OnlineID = onlineID;
+            OnlineID = onlineID ?? -1;
             Name = name;
             ShortName = shortName;
             Available = available;
diff --git a/osu.Game/Rulesets/RulesetInfo.cs b/osu.Game/Rulesets/RulesetInfo.cs
index ca6a083a58..8cd3fa8c63 100644
--- a/osu.Game/Rulesets/RulesetInfo.cs
+++ b/osu.Game/Rulesets/RulesetInfo.cs
@@ -57,7 +57,7 @@ namespace osu.Game.Rulesets
 
         #region Implementation of IHasOnlineID
 
-        public int? OnlineID => ID;
+        public int OnlineID => ID ?? -1;
 
         #endregion
     }