discourse/app/serializers/post_action_user_serializer.rb

class PostActionUserSerializer < BasicUserSerializer
  attributes :post_url,
             :username_lower

  def id
    object.user.id
  end

  def username
    object.user.username
  end

  def username_lower
    object.user.username_lower
  end

  def avatar_template
    object.user.avatar_template
  end

  def post_url
    object.related_post.url if object.related_post_id && object.related_post
  end

end