A vote turned the other way replaces the first

Lemmy turns an upvote into a downvote with a Dislike alone (and back with a
Like), so the like stayed counted next to the downvote. Now an account has
one vote on a post: a Dislike takes its like away, a Like its downvote.

The Lemmy scenario's relayed votes were never failing for that reason only:
Lemmy 1.0 sends what it queued every 30 seconds, and the check gave up after
30. It waits a minute now, and both votes are plain checks: 22 pass, the
moderator's removal stays an expected failure (P7).

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

PrivaPub.Tests/Federation/InteractionTests.cs

@@ -95,17 +95,43 @@ namespace PrivaPub.Tests.Federation
 				["id"] = $"{Origin(carol)}/reactions/{Guid.NewGuid():N}", ["type"] = "EmojiReact", ["actor"] = carol.Id, ["object"] = post.ObjectURI,
 				["content"] = "🔥"
 			});
+			Assert.Equal(1, (await DB.Default.Find<Post>().OneAsync(post.ID, token)).FavouritesCount);
 			await _harness.Deliver(carol, "/human-centipede", new JsonObject
 			{
 				["id"] = $"{Origin(carol)}/dislikes/{Guid.NewGuid():N}", ["type"] = "Dislike", ["actor"] = carol.Id, ["object"] = post.ObjectURI
 			});
 
+			// the downvote counts, and takes the place of carol's like (one vote each, as Lemmy votes)
 			var stored = await DB.Default.Find<Post>().OneAsync(post.ID, token);
-			Assert.Equal(1, stored.FavouritesCount);
+			Assert.Equal(0, stored.FavouritesCount);
 			Assert.Equal(1, stored.DownvotesCount);
 			Assert.True(await DB.Default.Find<Reaction>().Match(r => r.PostId == post.ID && r.ActorURI == carol.Id && r.Emoji == "🔥").ExecuteAnyAsync(token));
 		}
 
+		[Fact]
+		public async Task A_vote_turned_the_other_way_replaces_the_first_as_lemmy_sends_it()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var (_, post) = await LocalPost();
+			var voter = new RemoteActor(_harness.Peer, "voter");
+			var other = new RemoteActor(_harness.Peer, "other");
+			JsonObject Vote(RemoteActor actor, string type) => new()
+			{
+				["id"] = $"{Origin(actor)}/{type.ToLowerInvariant()}s/{Guid.NewGuid():N}", ["type"] = type, ["actor"] = actor.Id, ["object"] = post.ObjectURI
+			};
+
+			await _harness.Deliver(other, "/human-centipede", Vote(other, "Like"));
+			await _harness.Deliver(voter, "/human-centipede", Vote(voter, "Dislike"));
+			await _harness.Deliver(voter, "/human-centipede", Vote(voter, "Like"));
+			var up = await DB.Default.Find<Post>().OneAsync(post.ID, token);
+			Assert.Equal((2, 0), (up.FavouritesCount, up.DownvotesCount));
+
+			await _harness.Deliver(voter, "/human-centipede", Vote(voter, "Dislike"));
+			var down = await DB.Default.Find<Post>().OneAsync(post.ID, token);
+			Assert.Equal((1, 1), (down.FavouritesCount, down.DownvotesCount));
+			Assert.False(await DB.Default.Find<Favourite>().Match(f => f.PostId == post.ID && f.ActorURI == voter.Id).ExecuteAnyAsync(token));
+		}
+
 		[Fact]
 		public async Task A_circle_member_likes_a_circle_post_and_an_outsider_cannot()
 		{