Load mentions for users that left the guild

DiscordChatExporter.Core/Discord/Data/Channel.cs

@@ -23,19 +23,6 @@ public partial record Channel(
 
 public partial record Channel
 {
-    private static ChannelCategory GetFallbackCategory(ChannelKind channelKind) => new(
-        Snowflake.Zero,
-        channelKind switch
-        {
-            ChannelKind.GuildTextChat => "Text",
-            ChannelKind.DirectTextChat => "Private",
-            ChannelKind.DirectGroupTextChat => "Group",
-            ChannelKind.GuildNews => "News",
-            _ => "Default"
-        },
-        null
-    );
-
     public static Channel Parse(JsonElement json, ChannelCategory? categoryHint = null, int? positionHint = null)
     {
         var id = json.GetProperty("id").GetNonWhiteSpaceString().Pipe(Snowflake.Parse);
@@ -45,7 +32,7 @@ public partial record Channel
             json.GetPropertyOrNull("guild_id")?.GetNonWhiteSpaceStringOrNull()?.Pipe(Snowflake.Parse) ??
             Guild.DirectMessages.Id;
 
-        var category = categoryHint ?? GetFallbackCategory(kind);
+        var category = categoryHint ?? ChannelCategory.CreateDefault(kind);
 
         var name =
             // Guild channel

DiscordChatExporter.Core/Discord/Data/ChannelCategory.cs

@@ -7,6 +7,19 @@ namespace DiscordChatExporter.Core.Discord.Data;
 
 public record ChannelCategory(Snowflake Id, string Name, int? Position) : IHasId
 {
+    public static ChannelCategory CreateDefault(ChannelKind channelKind) => new(
+        Snowflake.Zero,
+        channelKind switch
+        {
+            ChannelKind.GuildTextChat => "Text",
+            ChannelKind.DirectTextChat => "Private",
+            ChannelKind.DirectGroupTextChat => "Group",
+            ChannelKind.GuildNews => "News",
+            _ => "Default"
+        },
+        null
+    );
+
     public static ChannelCategory Parse(JsonElement json, int? positionHint = null)
     {
         var id = json.GetProperty("id").GetNonWhiteSpaceString().Pipe(Snowflake.Parse);

DiscordChatExporter.Core/Discord/Data/Member.cs

@@ -20,6 +20,8 @@ public partial record Member(
 
 public partial record Member
 {
+    public static Member CreateDefault(User user) => new(user, null, null, Array.Empty<Snowflake>());
+
     public static Member Parse(JsonElement json, Snowflake? guildId = null)
     {
         var user = json.GetProperty("user").Pipe(User.Parse);

DiscordChatExporter.Core/Discord/DiscordClient.cs

@@ -150,6 +150,14 @@ public class DiscordClient
             : null;
     }
 
+    public async ValueTask<User?> TryGetUserAsync(
+        Snowflake userId,
+        CancellationToken cancellationToken = default)
+    {
+        var response = await TryGetJsonResponseAsync($"users/{userId}", cancellationToken);
+        return response?.Pipe(User.Parse);
+    }
+
     public async IAsyncEnumerable<Guild> GetUserGuildsAsync(
         [EnumeratorCancellation] CancellationToken cancellationToken = default)
     {

DiscordChatExporter.Core/Exporting/ChannelExporter.cs

@@ -34,7 +34,7 @@ public class ChannelExporter
         {
             // Resolve members for referenced users
             foreach (var user in message.GetReferencedUsers())
-                await context.PopulateMemberAsync(user.Id, cancellationToken);
+                await context.PopulateMemberAsync(user, cancellationToken);
 
             // Export the message
             if (request.MessageFilter.IsMatch(message))

DiscordChatExporter.Core/Exporting/ExportContext.cs

@@ -20,6 +20,7 @@ internal class ExportContext
     private readonly ExportAssetDownloader _assetDownloader;
 
     public DiscordClient Discord { get; }
+
     public ExportRequest Request { get; }
 
     public ExportContext(DiscordClient discord,
@@ -43,18 +44,35 @@ internal class ExportContext
             _roles[role.Id] = role;
     }
 
-    // Because members are not pulled in bulk, we need to populate them on demand
-    public async ValueTask PopulateMemberAsync(Snowflake id, CancellationToken cancellationToken = default)
+    // Because members cannot be pulled in bulk, we need to populate them on demand
+    private async ValueTask PopulateMemberAsync(
+        Snowflake id,
+        User? fallbackUser,
+        CancellationToken cancellationToken = default)
     {
         if (_members.ContainsKey(id))
             return;
 
         var member = await Discord.TryGetGuildMemberAsync(Request.Guild.Id, id, cancellationToken);
 
+        // User may have left the guild since they were mentioned
+        if (member is null)
+        {
+            var user = fallbackUser ?? await Discord.TryGetUserAsync(id, cancellationToken);
+            if (user is not null)
+                member = Member.CreateDefault(user);
+        }
+
         // Store the result even if it's null, to avoid re-fetching non-existing members
         _members[id] = member;
     }
 
+    public async ValueTask PopulateMemberAsync(Snowflake id, CancellationToken cancellationToken = default) =>
+        await PopulateMemberAsync(id, null, cancellationToken);
+
+    public async ValueTask PopulateMemberAsync(User user, CancellationToken cancellationToken = default) =>
+        await PopulateMemberAsync(user.Id, user, cancellationToken);
+
     public string FormatDate(DateTimeOffset instant) => Request.DateFormat switch
     {
         "unix" => instant.ToUnixTimeSeconds().ToString(),