Add likes to activitypub object representation

Top level of the likes OrderedCollection is inlined to get immediate
access to totalItems. Because the count can be returned without scanning
the database for like activities the extra query is saved when the
client only wants to display the total.

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -629,6 +629,7 @@ def prepare_object(object) do
     |> add_mention_tags
     |> add_emoji_tags
     |> add_attributed_to
+    |> add_likes
     |> prepare_attachments
     |> set_conversation
     |> set_reply_to_uri
@@ -788,6 +789,22 @@ def add_attributed_to(object) do
     |> Map.put("attributedTo", attributedTo)
   end
 
+  def add_likes(%{"id" => id, "like_count" => likes} = object) do
+    likes = %{
+      "id" => "#{id}/likes",
+      "first" => "#{id}/likes?page=1",
+      "type" => "OrderedCollection",
+      "totalItems" => likes
+    }
+
+    object
+    |> Map.put("likes", likes)
+  end
+
+  def add_likes(object) do
+    object
+  end
+
   def prepare_attachments(object) do
     attachments =
       (object["attachment"] || [])
@@ -803,7 +820,6 @@ def prepare_attachments(object) do
   defp strip_internal_fields(object) do
     object
     |> Map.drop([
-      "likes",
       "like_count",
       "announcements",
       "announcement_count",

test/web/activity_pub/transmogrifier_test.exs

@@ -829,7 +829,6 @@ test "it strips internal fields" do
       assert length(modified["object"]["tag"]) == 2
 
       assert is_nil(modified["object"]["emoji"])
-      assert is_nil(modified["object"]["likes"])
       assert is_nil(modified["object"]["like_count"])
       assert is_nil(modified["object"]["announcements"])
       assert is_nil(modified["object"]["announcement_count"])
@@ -844,12 +843,19 @@ test "it strips internal fields of article" do
       assert length(modified["object"]["tag"]) == 2
 
       assert is_nil(modified["object"]["emoji"])
-      assert is_nil(modified["object"]["likes"])
       assert is_nil(modified["object"]["like_count"])
       assert is_nil(modified["object"]["announcements"])
       assert is_nil(modified["object"]["announcement_count"])
       assert is_nil(modified["object"]["context_id"])
     end
+
+    test "it adds like collection to object" do
+      activity = insert(:note_activity)
+      {:ok, modified} = Transmogrifier.prepare_outgoing(activity.data)
+
+      assert modified["object"]["likes"]["type"] == "OrderedCollection"
+      assert modified["object"]["likes"]["totalItems"] == 0
+    end
   end
 
   describe "user upgrade" do