discourse/app/serializers/topic_list_item_serializer.rb

class TopicListItemSerializer < ListableTopicSerializer

  attributes :views,
             :like_count,
             :has_summary,
             :archetype,
             :last_poster_username,
             :category_id,
             :op_like_count,
             :pinned_globally

  has_many :posters, serializer: TopicPosterSerializer, embed: :objects
  has_many :participants, serializer: TopicPosterSerializer, embed: :objects

  def posters
    object.posters || []
  end

  def op_like_count
    object.first_post && object.first_post.like_count
  end

  def last_poster_username
    posters.find { |poster| poster.user.id == object.last_post_user_id }.try(:user).try(:username)
  end

  def participants
    object.participants_summary || []
  end

  def include_participants?
    object.private_message?
  end

  def include_op_like_count?
    # PERF: long term we probably want a cheaper way of looking stuff up
    # this is rather odd code, but we need to have op_likes loaded somehow
    # simplest optimisation is adding a cache column on topic.
    object.association(:first_post).loaded?
  end

end