P6: emoji reactions in and out

- Incoming reactions in all three shapes: Misskey and Sharkey Likes carrying an emoji (`content` or
  `_misskey_reaction`), Pleroma, Akkoma and Iceshrimp.NET EmojiReacts, and their Undo. Unicode reactions are one
  grapheme; custom ones need their Emoji tag and keep its image; `:name@host:` is read as `name`. A Like whose
  content is a heart stays a favourite.
- Reactions are kept per account and emoji on our posts and on remote ones we hold, and the author of a local post
  gets a `pleroma:emoji_reaction` notification with the emoji.
- Personas react through Pleroma's API (PUT/DELETE /api/v1/pleroma/statuses/:id/reactions/:emoji, GET for the list),
  which sends an EmojiReact (or its Undo) to the post's author.
- Statuses carry `emoji_reactions` (read by Phanpy) and `pleroma.emoji_reactions`, with counts and whether the viewer
  reacted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/Api/Mastodon/Controllers/ReactionsController.cs

@@ -0,0 +1,76 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+
+using MongoDB.Entities;
+
+using PrivaPub.Api.Mastodon.Infrastructure;
+using PrivaPub.Api.Mastodon.Mappers;
+using PrivaPub.Domain.Privacy;
+using PrivaPub.Domain.Social;
+using PrivaPub.Models.Post;
+using PrivaPub.Models.Social;
+using PrivaPub.StaticServices;
+
+using PostEntity = PrivaPub.Models.Post.Post;
+
+namespace PrivaPub.Api.Mastodon.Controllers
+{
+	public class ReactionsController : MastodonController
+	{
+		readonly DbEntities _dbEntities;
+		readonly MastodonMapper _mapper;
+		readonly IReactions _reactions;
+
+		public ReactionsController(DbEntities dbEntities, MastodonMapper mapper, IReactions reactions)
+		{
+			_dbEntities = dbEntities;
+			_mapper = mapper;
+			_reactions = reactions;
+		}
+
+		[HttpGet("/api/v1/pleroma/statuses/{id}/reactions"), Scope("read:statuses", requiresUser: false), AllowAnonymous]
+		public async Task<IActionResult> List(string id, CancellationToken token)
+		{
+			var post = await Visible(id, token);
+			if (post == default)
+				return NotFoundError();
+			var reactions = await DB.Default.Find<Reaction>().Match(r => r.PostId == post.ID).ExecuteAsync(token);
+			var accounts = await _mapper.Accounts(reactions.Select(r => r.AccountId), token);
+			return Json(reactions.GroupBy(r => r.Emoji).Select(group => new
+			{
+				name = group.Key,
+				count = group.Count(),
+				me = MyId != default && group.Any(r => r.AccountId == MyId),
+				url = _mapper.ProxiedUrl(group.First().EmojiURL),
+				accounts = group.Select(r => accounts.GetValueOrDefault(r.AccountId)).Where(a => a != default).ToList()
+			}).OrderByDescending(r => r.count).ToList());
+		}
+
+		[HttpPut("/api/v1/pleroma/statuses/{id}/reactions/{emoji}"), Scope("write:favourites")]
+		public async Task<IActionResult> React(string id, string emoji, CancellationToken token)
+		{
+			var post = await Visible(id, token);
+			if (post == default)
+				return NotFoundError();
+			if (!await _reactions.React(Me, post, Uri.UnescapeDataString(emoji), token))
+				return Error(StatusCodes.Status422UnprocessableEntity, "Validation failed: That emoji cannot be used here");
+			return Json(await _mapper.Status(post, MyId, token));
+		}
+
+		[HttpDelete("/api/v1/pleroma/statuses/{id}/reactions/{emoji}"), Scope("write:favourites")]
+		public async Task<IActionResult> Unreact(string id, string emoji, CancellationToken token)
+		{
+			var post = await Visible(id, token);
+			if (post == default)
+				return NotFoundError();
+			await _reactions.Unreact(Me, post, Uri.UnescapeDataString(emoji), token);
+			return Json(await _mapper.Status(post, MyId, token));
+		}
+
+		async Task<PostEntity> Visible(string id, CancellationToken token)
+		{
+			var post = await _dbEntities.Posts.Match(p => p.ID == id && !p.DeletedAt.HasValue && p.ReblogOfPostId == null).ExecuteFirstAsync(token);
+			return post != default && post.Visibility != PostVisibility.LocalGeo && await VisibilityPolicy.CanSee(post, MyId, token) ? post : default;
+		}
+	}
+}

PrivaPub/Api/Mastodon/Controllers/TimelinesController.cs

@@ -163,7 +163,8 @@ namespace PrivaPub.Api.Mastodon.Controllers
 			[NotificationType.Favourite] = "favourite",
 			[NotificationType.Reblog] = "reblog",
 			[NotificationType.Update] = "update",
-			[NotificationType.Poll] = "poll"
+			[NotificationType.Poll] = "poll",
+			[NotificationType.Reaction] = "pleroma:emoji_reaction"
 		};
 
 		readonly MastodonMapper _mapper;
@@ -234,7 +235,9 @@ namespace PrivaPub.Api.Mastodon.Controllers
 					CreatedAt = MastodonJson.Time(n.CreatedAt),
 					GroupKey = $"ungrouped-{n.ID}",
 					Account = accounts[n.FromAccountId],
-					Status = n.PostId == default ? default : statuses[n.PostId]
+					Status = n.PostId == default ? default : statuses[n.PostId],
+					Emoji = n.Emoji == default ? default : n.EmojiURL == default ? n.Emoji : $":{n.Emoji}:",
+					EmojiUrl = _mapper.ProxiedUrl(n.EmojiURL)
 				})
 				.ToList();
 		}

PrivaPub/Api/Mastodon/Entities/Entities.cs

@@ -161,6 +161,20 @@ namespace PrivaPub.Api.Mastodon.Entities
 		public int Down { get; set; }
 	}
 
+	public class EmojiReactionEntity
+	{
+		public string Name { get; set; }
+		public int Count { get; set; }
+		public bool Me { get; set; }
+		public string Url { get; set; }
+		public string StaticUrl { get; set; }
+	}
+
+	public class PleromaStatus
+	{
+		public List<EmojiReactionEntity> EmojiReactions { get; set; } = new();
+	}
+
 	public class PollEntity
 	{
 		public string Id { get; set; }
@@ -237,6 +251,8 @@ namespace PrivaPub.Api.Mastodon.Entities
 		public string Text { get; set; }
 		public List<object> Filtered { get; set; } = new();
 		public PrivaPubStatus Privapub { get; set; }
+		public List<EmojiReactionEntity> EmojiReactions { get; set; } = new();
+		public PleromaStatus Pleroma { get; set; }
 	}
 
 	public class MediaAttachment
@@ -305,6 +321,8 @@ namespace PrivaPub.Api.Mastodon.Entities
 		public string GroupKey { get; set; }
 		public Account Account { get; set; }
 		public Status Status { get; set; }
+		public string Emoji { get; set; }
+		public string EmojiUrl { get; set; }
 	}
 
 	public class Context

PrivaPub/Api/Mastodon/Mappers/MastodonMapper.cs

@@ -36,6 +36,8 @@ namespace PrivaPub.Api.Mastodon.Mappers
 
 		string Proxied(string url) => _proxy == default ? url : _proxy.Wrap(url);
 
+		public string ProxiedUrl(string url) => Proxied(url);
+
 		string MissingAvatar => $"{_localActors.BaseAddress}/media/missing-avatar.png";
 		string MissingHeader => $"{_localActors.BaseAddress}/media/missing-header.png";
 
@@ -166,6 +168,19 @@ namespace PrivaPub.Api.Mastodon.Mappers
 				? new HashSet<string>()
 				: (await DB.Default.Find<Bookmark>().Match(b => b.AvatarId == viewerId && ids.Contains(b.PostId)).ExecuteAsync(token)).Select(b => b.PostId).ToHashSet();
 			var pinned = (await DB.Default.Find<Pin>().Match(p => ids.Contains(p.PostId)).ExecuteAsync(token)).Select(p => p.PostId).ToHashSet();
+			var reactions = (await DB.Default.Find<Reaction>().Match(r => ids.Contains(r.PostId)).ExecuteAsync(token))
+				.GroupBy(r => r.PostId)
+				.ToDictionary(g => g.Key, g => g.GroupBy(r => r.Emoji)
+					.Select(e => new EmojiReactionEntity
+					{
+						Name = e.Key,
+						Count = e.Count(),
+						Me = viewerId != default && e.Any(r => r.AccountId == viewerId),
+						Url = Proxied(e.First().EmojiURL),
+						StaticUrl = Proxied(e.First().EmojiURL)
+					})
+					.OrderByDescending(e => e.Count)
+					.ToList());
 			var pollIds = all.Where(p => p.Poll != default).Select(p => p.ID).ToList();
 			var ownVotes = viewerId == default || pollIds.Count == 0
 				? new Dictionary<string, List<int>>()
@@ -210,6 +225,8 @@ namespace PrivaPub.Api.Mastodon.Mappers
 					Poll = Poll(post, viewerId, ownVotes.GetValueOrDefault(post.ID)),
 					Emojis = Emojis(post.Emojis),
 					Privapub = Extension(post),
+					EmojiReactions = reactions.GetValueOrDefault(post.ID) ?? new(),
+					Pleroma = new PleromaStatus { EmojiReactions = reactions.GetValueOrDefault(post.ID) ?? new() },
 					Mentions = post.Mentions.Select(m => Mention(m, mentionAccounts)).Where(m => m != default).ToList(),
 					Tags = post.Tags.Select(t => new StatusTag { Name = t, Url = ActivityPubRenderer.TagUrl(_localActors.BaseAddress, t) }).ToList()
 				};

PrivaPub/Domain/Social/Notifications.cs

@@ -8,7 +8,7 @@ namespace PrivaPub.Domain.Social
 	public static class Notifications
 	{
 		public static async Task Add(string avatarId, NotificationType type, string fromAccountId, string fromActorUri, string postId,
-			CancellationToken token)
+			CancellationToken token, string emoji = default, string emojiUrl = default)
 		{
 			if (string.IsNullOrEmpty(avatarId) || avatarId == fromAccountId && type != NotificationType.Poll)
 				return;
@@ -23,7 +23,9 @@ namespace PrivaPub.Domain.Social
 					FromAccountId = fromAccountId,
 					FromActorURI = fromActorUri,
 					PostId = postId,
-					DedupeKey = $"{type}|{avatarId}|{fromActorUri}|{postId}"
+					Emoji = emoji,
+					EmojiURL = emojiUrl,
+					DedupeKey = emoji == default ? $"{type}|{avatarId}|{fromActorUri}|{postId}" : $"{type}|{avatarId}|{fromActorUri}|{postId}|{emoji}"
 				}, token);
 			}
 			catch (MongoWriteException ex) when (ex.WriteError?.Category == ServerErrorCategory.DuplicateKey)

PrivaPub/Domain/Social/Reactions.cs

@@ -0,0 +1,179 @@
+using MongoDB.Driver;
+using MongoDB.Entities;
+
+using PrivaPub.Federation.Actors;
+using PrivaPub.Federation.Inbox.Handlers;
+using PrivaPub.Federation.Objects;
+using PrivaPub.Federation.Outbox;
+using PrivaPub.Federation.Rendering;
+using PrivaPub.Models.Social;
+using PrivaPub.Models.User;
+using PrivaPub.StaticServices;
+
+using System.Globalization;
+using System.Text.Json.Nodes;
+
+using PostEntity = PrivaPub.Models.Post.Post;
+
+namespace PrivaPub.Domain.Social
+{
+	public interface IReactions
+	{
+		Task Receive(PostEntity post, ForeignAvatar actor, string content, JsonNode tags, string activityId, CancellationToken token);
+		Task<bool> Withdraw(ForeignAvatar actor, string activityId, string objectUri, string content, CancellationToken token);
+		Task<bool> React(LocalActor me, PostEntity post, string emoji, CancellationToken token);
+		Task<bool> Unreact(LocalActor me, PostEntity post, string emoji, CancellationToken token);
+	}
+
+	public class Reactions : IReactions
+	{
+		static readonly string[] Hearts = { "❤", "❤️", "♥", "♥️" };
+
+		readonly DbEntities _dbEntities;
+		readonly IDeliveryService _delivery;
+
+		public Reactions(DbEntities dbEntities, IDeliveryService delivery)
+		{
+			_dbEntities = dbEntities;
+			_delivery = delivery;
+		}
+
+		public static bool IsHeart(string content) => content == default || Hearts.Contains(content.Trim());
+
+		public async Task Receive(PostEntity post, ForeignAvatar actor, string content, JsonNode tags, string activityId, CancellationToken token)
+		{
+			if (!LikeHandler.MaySee(post, actor) || Normalise(content, ObjectShapes.Emojis(tags)) is not { } emoji)
+				return;
+			try
+			{
+				await DB.Default.SaveAsync(new Reaction
+				{
+					PostId = post.ID, AccountId = actor.ID, ActorURI = actor.ActorURI, Emoji = emoji.Name, EmojiURL = emoji.Url, ActivityURI = activityId
+				}, token);
+			}
+			catch (MongoWriteException ex) when (ex.WriteError?.Category == ServerErrorCategory.DuplicateKey)
+			{
+				return;
+			}
+			if (!post.IsFederatedCopy)
+				await Notifications.Add(post.GroupUserId, NotificationType.Reaction, actor.ID, actor.ActorURI, post.ID, token, emoji.Name, emoji.Url);
+		}
+
+		public async Task<bool> Withdraw(ForeignAvatar actor, string activityId, string objectUri, string content, CancellationToken token)
+		{
+			var reaction = activityId == default
+				? default
+				: await DB.Default.Find<Reaction>().Match(r => r.ActivityURI == activityId && r.ActorURI == actor.ActorURI).ExecuteFirstAsync(token);
+			if (reaction == default && objectUri != default && Normalise(content, new()) is { } emoji)
+			{
+				var post = await _dbEntities.Posts.Match(p => p.ObjectURI == objectUri).ExecuteFirstAsync(token);
+				reaction = post == default
+					? default
+					: await DB.Default.Find<Reaction>().Match(r => r.PostId == post.ID && r.ActorURI == actor.ActorURI && r.Emoji == emoji.Name).ExecuteFirstAsync(token);
+			}
+			if (reaction == default)
+				return false;
+			await DB.Default.DeleteAsync<Reaction>(reaction.ID);
+			return true;
+		}
+
+		public async Task<bool> React(LocalActor me, PostEntity post, string emoji, CancellationToken token)
+		{
+			var known = post.Emojis.Select(e => (e.Shortcode, e.URL))
+				.Concat((await DB.Default.Find<Reaction>().Match(r => r.PostId == post.ID && r.EmojiURL != null).ExecuteAsync(token)).Select(r => (r.Emoji, r.EmojiURL)))
+				.Select(e => new Models.Post.CustomEmoji { Shortcode = e.Item1, URL = e.Item2 })
+				.ToList();
+			if (Normalise(emoji, known) is not { } picked)
+				return false;
+			var reaction = new Reaction
+			{
+				PostId = post.ID, AccountId = me.Id, ActorURI = me.Uri, IsLocal = true, Emoji = picked.Name, EmojiURL = picked.Url,
+				ActivityURI = me.ActivityUri($"react-{post.ID}-{Guid.NewGuid():N}")
+			};
+			try
+			{
+				await DB.Default.SaveAsync(reaction, token);
+			}
+			catch (MongoWriteException ex) when (ex.WriteError?.Category == ServerErrorCategory.DuplicateKey)
+			{
+				return true;
+			}
+			if (!post.IsFederatedCopy)
+				await Notifications.Add(post.GroupUserId, NotificationType.Reaction, me.Id, me.Uri, post.ID, token, picked.Name, picked.Url);
+			else
+				await Tell(me, post, Activity(me, post, reaction), token);
+			return true;
+		}
+
+		public async Task<bool> Unreact(LocalActor me, PostEntity post, string emoji, CancellationToken token)
+		{
+			var name = Normalise(emoji, post.Emojis.ToList(), requireKnownCustom: false)?.Name;
+			var reaction = name == default
+				? default
+				: await DB.Default.Find<Reaction>().Match(r => r.PostId == post.ID && r.ActorURI == me.Uri && r.Emoji == name).ExecuteFirstAsync(token);
+			if (reaction == default)
+				return false;
+			await DB.Default.DeleteAsync<Reaction>(reaction.ID);
+			if (post.IsFederatedCopy)
+				await Tell(me, post, new JsonObject
+				{
+					["@context"] = ActivityPubRenderer.Context(),
+					["id"] = me.ActivityUri($"undo-{reaction.ID}"),
+					["type"] = "Undo",
+					["actor"] = me.Uri,
+					["object"] = Activity(me, post, reaction)
+				}, token);
+			return true;
+		}
+
+		static JsonObject Activity(LocalActor me, PostEntity post, Reaction reaction)
+		{
+			var react = new JsonObject
+			{
+				["@context"] = ActivityPubRenderer.Context(),
+				["id"] = reaction.ActivityURI,
+				["type"] = "EmojiReact",
+				["actor"] = me.Uri,
+				["object"] = post.ObjectURI,
+				["to"] = new JsonArray(post.ActorURI),
+				["content"] = reaction.EmojiURL == default ? reaction.Emoji : $":{reaction.Emoji}:"
+			};
+			if (reaction.EmojiURL != default)
+				react["tag"] = new JsonArray(new JsonObject
+				{
+					["type"] = "Emoji",
+					["name"] = $":{reaction.Emoji}:",
+					["icon"] = new JsonObject { ["type"] = "Image", ["url"] = reaction.EmojiURL }
+				});
+			return react;
+		}
+
+		async Task Tell(LocalActor me, PostEntity post, JsonObject activity, CancellationToken token)
+		{
+			var author = await _dbEntities.ForeignAvatars.Match(f => f.ActorURI == post.ActorURI).ExecuteFirstAsync(token);
+			if (!string.IsNullOrEmpty(author?.InboxURL))
+				await _delivery.Enqueue(me, new[] { author.InboxURL }, activity, token);
+		}
+
+		public sealed record Picked(string Name, string Url);
+
+		public static Picked Normalise(string content, List<Models.Post.CustomEmoji> emojis, bool requireKnownCustom = true)
+		{
+			var text = content?.Trim();
+			if (string.IsNullOrEmpty(text) || text.Length > 100)
+				return default;
+			if (text.StartsWith(':') && text.EndsWith(':') && text.Length > 2)
+			{
+				var shortcode = text.Trim(':');
+				var bare = shortcode.Split('@')[0];
+				var emoji = emojis.FirstOrDefault(e => e.Shortcode == shortcode) ?? emojis.FirstOrDefault(e => e.Shortcode.Split('@')[0] == bare);
+				if (emoji == default)
+					return requireKnownCustom ? default : new Picked(bare, default);
+				return new Picked(bare, emoji.URL);
+			}
+			if (text.Any(c => c < 128 || char.IsWhiteSpace(c)) || new StringInfo(text).LengthInTextElements != 1)
+				return default;
+			return new Picked(text, default);
+		}
+	}
+}

PrivaPub/Federation/Inbox/Handlers/EmojiReactHandler.cs

@@ -0,0 +1,34 @@
+using PrivaPub.Domain.Social;
+using PrivaPub.Models.User;
+using PrivaPub.StaticServices;
+
+using System.Text.Json.Nodes;
+
+using static PrivaPub.Federation.Objects.ActivityJson;
+
+namespace PrivaPub.Federation.Inbox.Handlers
+{
+	public class EmojiReactHandler : IActivityHandler
+	{
+		readonly DbEntities _dbEntities;
+		readonly IReactions _reactions;
+
+		public EmojiReactHandler(DbEntities dbEntities, IReactions reactions)
+		{
+			_dbEntities = dbEntities;
+			_reactions = reactions;
+		}
+
+		public string Type => "EmojiReact";
+
+		public async Task Handle(JsonNode activity, ForeignAvatar actor, CancellationToken token)
+		{
+			var objectUri = Id(activity["object"]);
+			var post = objectUri == default
+				? default
+				: await _dbEntities.Posts.Match(p => p.ObjectURI == objectUri && !p.DeletedAt.HasValue && p.ReblogOfPostId == null).ExecuteFirstAsync(token);
+			if (post != default)
+				await _reactions.Receive(post, actor, Value(activity, "content"), activity["tag"], Id(activity), token);
+		}
+	}
+}

PrivaPub/Federation/Inbox/Handlers/LikeHandler.cs

@@ -18,10 +18,12 @@ namespace PrivaPub.Federation.Inbox.Handlers
 	public class LikeHandler : IActivityHandler
 	{
 		readonly DbEntities _dbEntities;
+		readonly IReactions _reactions;
 
-		public LikeHandler(DbEntities dbEntities)
+		public LikeHandler(DbEntities dbEntities, IReactions reactions)
 		{
 			_dbEntities = dbEntities;
+			_reactions = reactions;
 		}
 
 		public string Type => "Like";
@@ -34,6 +36,12 @@ namespace PrivaPub.Federation.Inbox.Handlers
 				: await _dbEntities.Posts.Match(p => p.ObjectURI == objectUri && !p.DeletedAt.HasValue && p.ReblogOfPostId == null).ExecuteFirstAsync(token);
 			if (post == default || !MaySee(post, actor))
 				return;
+			var content = Value(activity, "_misskey_reaction") ?? Value(activity, "content");
+			if (!Reactions.IsHeart(content))
+			{
+				await _reactions.Receive(post, actor, content, activity["tag"], Id(activity), token);
+				return;
+			}
 
 			try
 			{

PrivaPub/Federation/Inbox/Handlers/UndoHandler.cs

@@ -1,5 +1,6 @@
 using MongoDB.Entities;
 
+using PrivaPub.Domain.Social;
 using PrivaPub.Federation.Actors;
 using PrivaPub.Models.Federation;
 using PrivaPub.Models.Social;
@@ -17,11 +18,13 @@ namespace PrivaPub.Federation.Inbox.Handlers
 {
 	public class UndoHandler : IActivityHandler
 	{
+		readonly IReactions _reactions;
 		readonly DbEntities _dbEntities;
 		readonly ILocalActorService _localActors;
 
-		public UndoHandler(DbEntities dbEntities, ILocalActorService localActors)
+		public UndoHandler(DbEntities dbEntities, ILocalActorService localActors, IReactions reactions)
 		{
+			_reactions = reactions;
 			_dbEntities = dbEntities;
 			_localActors = localActors;
 		}
@@ -42,6 +45,9 @@ namespace PrivaPub.Federation.Inbox.Handlers
 				await UndoAnnounce(innerId, actor, token);
 			if (innerType is null or "Dislike")
 				await UndoDislike(inner, innerId, actor, token);
+			if (innerType is null or "Like" or "EmojiReact")
+				await _reactions.Withdraw(actor, innerId, inner is JsonObject ? Id(inner["object"]) : default,
+					inner is JsonObject ? Value(inner, "_misskey_reaction") ?? Value(inner, "content") : default, token);
 		}
 
 		async Task UndoDislike(JsonNode inner, string innerId, ForeignAvatar actor, CancellationToken token)

PrivaPub/Federation/Rendering/ActivityPubRenderer.cs

@@ -38,6 +38,9 @@ namespace PrivaPub.Federation.Rendering
 				["indexable"] = "toot:indexable",
 				["blurhash"] = "toot:blurhash",
 				["votersCount"] = "toot:votersCount",
+				["Emoji"] = "toot:Emoji",
+				["litepub"] = "http://litepub.social/ns#",
+				["EmojiReact"] = "litepub:EmojiReact",
 				["focalPoint"] = new JsonObject { ["@container"] = "@list", ["@id"] = "toot:focalPoint" },
 				["schema"] = "http://schema.org#",
 				["PropertyValue"] = "schema:PropertyValue",

PrivaPub/Infrastructure/Data/Indexes.cs

@@ -113,6 +113,9 @@ namespace PrivaPub.Infrastructure.Data
 			await DB.Default.Index<PollVote>().Key(v => v.PostId, KeyType.Ascending).Key(v => v.ActorURI, KeyType.Ascending).Key(v => v.Choice, KeyType.Ascending)
 				.Option(o => o.Unique = true).CreateAsync(token);
 			await DB.Default.Index<PollVote>().Key(v => v.PostId, KeyType.Ascending).Key(v => v.VoterAccountId, KeyType.Ascending).CreateAsync(token);
+			await DB.Default.Index<Reaction>().Key(r => r.PostId, KeyType.Ascending).Key(r => r.ActorURI, KeyType.Ascending).Key(r => r.Emoji, KeyType.Ascending)
+				.Option(o => o.Unique = true).CreateAsync(token);
+			await Plain<Reaction>(token, r => r.ActivityURI);
 		}
 
 		static async Task Unique<T>(System.Linq.Expressions.Expression<Func<T, object>> key, FilterDefinition<T> partial,

PrivaPub/Middleware/SocialPubConfigurations.cs

@@ -77,6 +77,8 @@ namespace PrivaPub.Middleware
 				.AddSingleton<IActivityHandler, UndoHandler>()
 				.AddSingleton<IActivityHandler, LikeHandler>()
 				.AddSingleton<IActivityHandler, DislikeHandler>()
+				.AddSingleton<IActivityHandler, EmojiReactHandler>()
+				.AddSingleton<IReactions, Reactions>()
 				.AddSingleton<IActivityHandler, JoinHandler>()
 				.AddSingleton<IActivityHandler, AnnounceHandler>()
 				.AddSingleton<IActivityHandler, FlagHandler>()

PrivaPub/Models/Social/Notification.cs

@@ -10,6 +10,8 @@ namespace PrivaPub.Models.Social
 		public string FromActorURI { get; set; }
 		public string PostId { get; set; }
 		public string DedupeKey { get; set; }
+		public string Emoji { get; set; }//a reaction's emoji
+		public string EmojiURL { get; set; }
 		public bool IsRead { get; set; }
 		public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
 	}
@@ -22,6 +24,7 @@ namespace PrivaPub.Models.Social
 		Favourite,
 		Reblog,
 		Update,
-		Poll
+		Poll,
+		Reaction
 	}
 }

PrivaPub/Models/Social/Reaction.cs

@@ -0,0 +1,16 @@
+using MongoDB.Entities;
+
+namespace PrivaPub.Models.Social
+{
+	public class Reaction : Entity
+	{
+		public string PostId { get; set; }
+		public string AccountId { get; set; }//Avatar.ID for a local persona, ForeignAvatar.ID for a remote account
+		public string ActorURI { get; set; }
+		public bool IsLocal { get; set; }
+		public string Emoji { get; set; }//a single Unicode emoji, or a custom emoji's shortcode without colons or host
+		public string EmojiURL { get; set; }//set for a custom emoji
+		public string ActivityURI { get; set; }
+		public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+	}
+}