3 files changed, 80 insertions, 1 deletions
diff --git a/lib/pleroma/activity.ex b/lib/pleroma/activity.ex
index 896cbb3c5..b7be7a800 100644
--- a/lib/pleroma/activity.ex
+++ b/lib/pleroma/activity.ex
@@ -329,4 +329,23 @@ defmodule Pleroma.Activity do
       _ -> nil
     end
   end
+
+  def replies(activity, opts \\ []) do
+    object = Object.normalize(activity)
+
+    query =
+      Activity
+      |> Queries.by_type("Create")
+      |> Queries.by_object_in_reply_to_id(object.data["id"], skip_preloading: true)
+      |> order_by([activity], asc: activity.id)
+
+    if opts[:self_only] do
+      where(query, [a], a.actor == ^activity.actor)
+    else
+      query
+    end
+  end
+
+  def self_replies(activity, opts \\ []),
+    do: replies(activity, Keyword.put(opts, :self_only, true))
 end
diff --git a/lib/pleroma/activity/queries.ex b/lib/pleroma/activity/queries.ex
index 79f305201..c17affec9 100644
--- a/lib/pleroma/activity/queries.ex
+++ b/lib/pleroma/activity/queries.ex
@@ -7,7 +7,7 @@ defmodule Pleroma.Activity.Queries do
   Contains queries for Activity.
   """
 
-  import Ecto.Query, only: [from: 2]
+  import Ecto.Query, only: [from: 2, where: 3]
 
   @type query :: Ecto.Queryable.t() | Activity.t()
 
@@ -63,6 +63,22 @@ defmodule Pleroma.Activity.Queries do
     )
   end
 
+  @spec by_object_id(query, String.t()) :: query
+  def by_object_in_reply_to_id(query, in_reply_to_id, opts \\ []) do
+    query =
+      if opts[:skip_preloading] do
+        Activity.with_joined_object(query)
+      else
+        Activity.with_preloaded_object(query)
+      end
+
+    where(
+      query,
+      [activity, object: o],
+      fragment("(?)->>'inReplyTo' = ?", o.data, ^to_string(in_reply_to_id))
+    )
+  end
+
   @spec by_type(query, String.t()) :: query
   def by_type(query \\ Activity, activity_type) do
     from(
diff --git a/lib/pleroma/web/activity_pub/transmogrifier.ex b/lib/pleroma/web/activity_pub/transmogrifier.ex
index 2b8bfc3bd..9e712ab75 100644
--- a/lib/pleroma/web/activity_pub/transmogrifier.ex
+++ b/lib/pleroma/web/activity_pub/transmogrifier.ex
@@ -903,6 +903,49 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
 
   def set_reply_to_uri(obj), do: obj
 
+  @doc """
+  Serialized Mastodon-compatible `replies` collection containing _self-replies_.
+  Based on Mastodon's ActivityPub::NoteSerializer#replies.
+  """
+  def set_replies(obj) do
+    limit = Pleroma.Config.get([:mastodon_compatibility, :federated_note_replies_limit], 0)
+
+    replies_uris =
+      with true <- limit > 0 || nil,
+           %Activity{} = activity <- Activity.get_create_by_object_ap_id(obj["id"]) do
+        activity
+        |> Activity.self_replies()
+        |> select([a], fragment("?->>'id'", a.data))
+        |> limit(^limit)
+        |> Repo.all()
+      end
+
+    set_replies(obj, replies_uris || [])
+  end
+
+  defp set_replies(obj, replies_uris) when replies_uris in [nil, []] do
+    obj
+  end
+
+  defp set_replies(obj, replies_uris) do
+    # Note: stubs (Mastodon doesn't make separate requests via those URIs in FetchRepliesService)
+    masto_replies_uri = nil
+    masto_replies_next_page_uri = nil
+
+    replies_collection = %{
+      "type" => "Collection",
+      "id" => masto_replies_uri,
+      "first" => %{
+        "type" => "Collection",
+        "part_of" => masto_replies_uri,
+        "items" => replies_uris,
+        "next" => masto_replies_next_page_uri
+      }
+    }
+
+    Map.merge(obj, %{"replies" => replies_collection})
+  end
+
   # Prepares the object of an outgoing create activity.
   def prepare_object(object) do
     object
@@ -914,6 +957,7 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
     |> prepare_attachments
     |> set_conversation
     |> set_reply_to_uri
+    |> set_replies
     |> strip_internal_fields
     |> strip_internal_tags
     |> set_type