fix: don't delete rows from messages table when the associated member is deleted

PluralKit.Core/Database/Migrations/24.sql

@@ -0,0 +1,10 @@
+-- schema version 24
+-- don't drop message rows when system/member are deleted
+
+alter table messages alter column member drop not null;
+alter table messages drop contstraint messages_member_fkey;
+alter table messages
+    add constraint messages_member_fkey
+    foreign key (member) references members(id) on delete set null;
+
+update info set schema_version = 24;

PluralKit.Core/Database/Repository/ModelRepository.Message.cs

@@ -29,9 +29,13 @@ public partial class ModelRepository
         FullMessage Mapper(PKMessage msg, PKMember member, PKSystem system) =>
             new() { Message = msg, System = system, Member = member };
 
+        var query = "select * from messages"
+            + " left join members on messages.member = members.id"
+            + " left join systems on members.system = systems.id"
+            + " where (mid = @Id or original_mid = @Id)";
+
         var result = await conn.QueryAsync<PKMessage, PKMember, PKSystem, FullMessage>(
-            "select messages.*, members.*, systems.* from messages, members, systems where (mid = @Id or original_mid = @Id) and messages.member = members.id and systems.id = members.system",
-            Mapper, new { Id = id });
+            query, Mapper, new { Id = id });
         return result.FirstOrDefault();
     }
 

PluralKit.Core/Dispatch/DispatchService.cs

@@ -133,7 +133,7 @@ public class DispatchService
         if (system.WebhookUrl == null)
             return;
 
-        var member = await repo.GetMember(newMessage.Member);
+        var member = await repo.GetMember(newMessage.Member!.Value);
 
         var fullMessage = new FullMessage
         {

PluralKit.Core/Models/PKMessage.cs

@@ -9,7 +9,7 @@ public class PKMessage
     public ulong Mid { get; set; }
     public ulong? Guild { get; set; } // null value means "no data" (ie. from before this field being added)
     public ulong Channel { get; set; }
-    public MemberId Member { get; set; }
+    public MemberId? Member { get; set; }
     public ulong Sender { get; set; }
     public ulong? OriginalMid { get; set; }
 }
@@ -17,8 +17,8 @@ public class PKMessage
 public class FullMessage
 {
     public PKMessage Message;
-    public PKMember Member;
-    public PKSystem System;
+    public PKMember? Member;
+    public PKSystem? System;
 
     public JObject ToJson(LookupContext ctx, APIVersion v)
     {
@@ -30,8 +30,8 @@ public class FullMessage
         o.Add("sender", Message.Sender.ToString());
         o.Add("channel", Message.Channel.ToString());
         o.Add("guild", Message.Guild?.ToString());
-        o.Add("system", System.ToJson(ctx, v));
-        o.Add("member", Member.ToJson(ctx, v: v));
+        o.Add("system", System?.ToJson(ctx, v));
+        o.Add("member", Member?.ToJson(ctx, v: v));
 
         return o;
     }