discourse/app/controllers/post_readers_controller.rb

# frozen_string_literal: true

class PostReadersController < ApplicationController
  requires_login

  def index
    post = Post.includes(topic: %i[topic_allowed_groups topic_allowed_users]).find(params[:id])
    ensure_can_see_readers!(post)

    readers = User
      .where(staged: false)
      .where.not(id: post.user_id)
      .joins(:topic_users)
      .where.not(topic_users: { last_read_post_number: nil })
      .where('topic_users.topic_id = ? AND topic_users.last_read_post_number >= ?', post.topic_id, post.post_number)

    if post.whisper?
      non_group_members = post.topic.topic_allowed_users.map(&:user_id)
      readers = readers.where.not(id: non_group_members)
    end

    readers = readers.map do |r|
      {
        id: r.id, avatar_template: r.avatar_template,
        username: r.username,
        username_lower: r.username_lower
      }
    end

    render_json_dump(post_readers: readers)
  end

  private

  def ensure_can_see_readers!(post)
    show_readers = GroupUser
      .where(user: current_user)
      .joins(:group)
      .where(groups: { id: post.topic.topic_allowed_groups.map(&:group_id), publish_read_state: true }).exists?

    raise Discourse::InvalidAccess unless show_readers
  end
end