Conversations page, and know what is unread

/api/v1/conversations answered one page, never unread, its read endpoint
did nothing, and DELETE was missing; each conversation cost a query per
member. Now each conversation keeps its newest post (DmGroup.LastPostId,
set as posts arrive, learnt once by migration _013) and pages by it as
Mastodon does, and each persona's ConversationState holds what it read and
what it took off its list:

- unread when someone else wrote last, after what the persona read;
- read marks it so, and writing in a conversation reads it;
- DELETE takes it off the list until a newer message brings it back.

The list reads its states, newest posts, members and accounts in a few
queries per page.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Domain/Statuses/ConversationStates.cs

@@ -0,0 +1,59 @@
+using MongoDB.Driver;
+using MongoDB.Entities;
+
+using PrivaPub.Models.Group;
+using PrivaPub.Models.Social;
+
+namespace PrivaPub.Domain.Statuses
+{
+	// A direct conversation's newest post (DmGroup.LastPostId, which pages the conversations list) and each persona's
+	// read and removed marks on it (ConversationState). A conversation is unread for a persona when its newest post is
+	// someone else's and newer than what the persona read; writing in it reads it.
+	public static class ConversationStates
+	{
+		public static async Task Posted(string conversationId, string postId, string authorAvatarId, CancellationToken token)
+		{
+			await DB.Default.Update<DmGroup>().MatchID(conversationId)
+				.Modify(g => g.UpdatedAt, DateTime.UtcNow)
+				.Modify(g => g.LastPostId, postId)
+				.ExecuteAsync(token);
+			if (authorAvatarId != default)
+				await Read(authorAvatarId, conversationId, postId, token);
+		}
+
+		public static Task Read(string avatarId, string conversationId, string postId, CancellationToken token) =>
+			Mark(avatarId, conversationId, s => s.LastReadPostId, postId, token);
+
+		public static Task Hide(string avatarId, string conversationId, string postId, CancellationToken token) =>
+			Mark(avatarId, conversationId, s => s.HiddenUpTo, postId, token);
+
+		public static bool IsUnread(ConversationState state, string lastPostId, string lastAuthorId, string avatarId) =>
+			lastPostId != default && lastAuthorId != avatarId && string.CompareOrdinal(lastPostId, state?.LastReadPostId) > 0;
+
+		public static bool IsHidden(ConversationState state, string lastPostId) =>
+			state?.HiddenUpTo != default && string.CompareOrdinal(lastPostId, state.HiddenUpTo) <= 0;
+
+		// the mark only moves forward; the first mark makes the state (two at once: the unique index keeps one, and the
+		// loser marks again)
+		static async Task Mark(string avatarId, string conversationId, System.Linq.Expressions.Expression<Func<ConversationState, string>> field,
+			string postId, CancellationToken token)
+		{
+			if (postId == default)
+				return;
+			Task Write() => DB.Default.Update<ConversationState>()
+				.Match(s => s.AvatarId == avatarId && s.ConversationId == conversationId)
+				.Modify(b => b.Max(field, postId))
+				.Modify(s => s.UpdatedAt, DateTime.UtcNow)
+				.Option(o => o.IsUpsert = true)
+				.ExecuteAsync(token);
+			try
+			{
+				await Write();
+			}
+			catch (MongoWriteException ex) when (ex.WriteError?.Category == ServerErrorCategory.DuplicateKey)
+			{
+				await Write();
+			}
+		}
+	}
+}

PrivaPub/Domain/Statuses/StatusService.cs

@@ -237,7 +237,7 @@ namespace PrivaPub.Domain.Statuses
 				var note = ActivityPubRenderer.DirectNote(post, author, recipients.Select(r => (r.Uri, r.Handle)).ToList(), dmGroup.ConversationURI);
 				create = ActivityPubRenderer.Create(author, note, $"create-{post.ID}");
 				post.To = Strings(note["to"]);
-				await DB.Default.Update<DmGroup>().MatchID(dmGroup.ID).Modify(g => g.UpdatedAt, DateTime.UtcNow).ExecuteAsync(token);
+				await ConversationStates.Posted(dmGroup.ID, post.ID, author.Id, token);
 			}
 			else if (!isLocalOnly)
 			{