Add new `MarkerTimelineSerializer` class

app/serializers/rest/marker_timeline_serializer.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class REST::MarkerTimelineSerializer < ActiveModel::Serializer
+  Marker::TIMELINES.each do |timeline|
+    has_one timeline.to_sym,
+            if: -> { timeline_present?(timeline) },
+            serializer: REST::MarkerSerializer
+  end
+
+  delegate :timeline_present?, to: :object
+end