Add featured hashtags as an ActivityPub collection (#11595)

app/serializers/activitypub/actor_serializer.rb

@@ -10,7 +10,7 @@ class ActivityPub::ActorSerializer < ActivityPub::Serializer
                      :discoverable, :olm
 
   attributes :id, :type, :following, :followers,
-             :inbox, :outbox, :featured,
+             :inbox, :outbox, :featured, :featured_tags,
              :preferred_username, :name, :summary,
              :url, :manually_approves_followers,
              :discoverable
@@ -81,6 +81,10 @@ class ActivityPub::ActorSerializer < ActivityPub::Serializer
     account_collection_url(object, :featured)
   end
 
+  def featured_tags
+    account_collection_url(object, :tags)
+  end
+
   def endpoints
     object
   end

app/serializers/activitypub/collection_serializer.rb

@@ -16,6 +16,8 @@ class ActivityPub::CollectionSerializer < ActivityPub::Serializer
       ActivityPub::NoteSerializer
     when 'Device'
       ActivityPub::DeviceSerializer
+    when 'FeaturedTag'
+      ActivityPub::HashtagSerializer
     when 'ActivityPub::CollectionPresenter'
       ActivityPub::CollectionSerializer
     when 'String'

app/serializers/activitypub/hashtag_serializer.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class ActivityPub::HashtagSerializer < ActivityPub::Serializer
+  include RoutingHelper
+
+  attributes :type, :href, :name
+
+  def type
+    'Hashtag'
+  end
+
+  def name
+    "##{object.name}"
+  end
+
+  def href
+    if object.class.name == 'FeaturedTag'
+      short_account_tag_url(object.account, object.tag)
+    else
+      tag_url(object)
+    end
+  end
+end