Add migration for previous incorrectly imported beatmaps

osu.Game/Migrations/20180628011956_RemoveNegativeSetIDs.cs

@@ -0,0 +1,19 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+namespace osu.Game.Migrations
+{
+    public partial class RemoveNegativeSetIDs : Migration
+    {
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            // There was a change that baetmaps were being loaded with "-1" online IDs, which is completely incorrect.
+            // This ensures there will not be unique key conflicts as a result of these incorrectly imported beatmaps.
+            migrationBuilder.Sql("UPDATE BeatmapSetInfo SET OnlineBeatmapSetID = null WHERE OnlineBeatmapSetID < 0");
+        }
+
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+
+        }
+    }
+}