btrc-hub/backend/app/controllers/theatres_controller.rb

class TheatresController < ApplicationController
  def show
    theatre = Theatre.find_by(id: params[:id])
    return head :not_found unless theatre

    render json: TheatreRepr.base(theatre)
  end

  def watching
    return head :unauthorized unless current_user

    theatre = Theatre.find_by(id: params[:id])
    return head :not_found unless theatre

    host_flg = false
    post_id = nil
    post_started_at = nil

    theatre.with_lock do
      TheatreWatchingUser.find_or_initialize_by(theatre:, user: current_user).tap {
        _1.expires_at = 30.seconds.from_now
      }.save!

      if (!(theatre.host_user_id?) ||
          !(theatre.watching_users.exists?(id: theatre.host_user_id)))
        theatre.update!(host_user_id: current_user.id)
      end

      host_flg = theatre.host_user_id == current_user.id
      post_id = theatre.current_post_id
      post_started_at = theatre.current_post_started_at
    end

    render json: {
        host_flg:, post_id:, post_started_at:,
        watching_users: theatre.watching_users.as_json(only: [:id, :name]) }
  end

  def next_post
    return head :unauthorized unless current_user

    theatre = Theatre.find_by(id: params[:id])
    return head :not_found unless theatre
    return head :forbidden if theatre.host_user != current_user

    post = Post.where("url LIKE '%nicovideo.jp%'")
           .or(Post.where("url LIKE '%youtube.com%'"))
           .order('RAND()')
           .first
    theatre.update!(current_post: post, current_post_started_at: Time.current)

    head :no_content
  end
end