discourse/app/serializers/category_detailed_serializer.rb

class CategoryDetailedSerializer < ApplicationSerializer

  attributes :id,
             :name,
             :color,
             :text_color,
             :slug,
             :topic_count,
             :topics_week,
             :topics_month,
             :topics_year,
             :description

  has_many :featured_users, serializer: BasicUserSerializer
  has_many :featured_topics, serializer: CategoryTopicSerializer, embed: :objects, key: :topics

  def topics_week
    object.topics_week || 0
  end

  def topics_month
    object.topics_month || 0
  end

  def topics_year
    object.topics_year || 0
  end

end