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

class PostVersionsController < ApplicationController
  def index
    post_id = params[:post].presence
    tag_id = params[:tag].presence
    page = (params[:page].presence || 1).to_i
    limit = (params[:limit].presence || 20).to_i

    page = 1 if page < 1
    limit = 1 if limit < 1

    offset = (page - 1) * limit

    tag_name =
        if tag_id
          TagName.joins(:tag).find_by(tag: { id: tag_id })
        end
    return render json: { versions: [], count: 0 } if tag_id && tag_name.blank?

    q = PostVersion.joins(<<~SQL.squish)
          LEFT JOIN
            post_versions prev
            ON
              prev.post_id = post_versions.post_id
              AND prev.version_no = post_versions.version_no - 1
        SQL
        .select('post_versions.*', 'prev.title AS prev_title', 'prev.url AS prev_url',
                'prev.thumbnail_base AS prev_thumbnail_base', 'prev.tags AS prev_tags',
                'prev.original_created_from AS prev_original_created_from',
                'prev.original_created_before AS prev_original_created_before')
    q = q.where('post_versions.post_id = ?', post_id) if post_id
    if tag_name
      escaped = ActiveRecord::Base.sanitize_sql_like(tag_name.name)
      q = q.where(("CONCAT(' ', post_versions.tags, ' ') LIKE :kw " +
                   "OR CONCAT(' ', prev.tags, ' ') LIKE :kw"),
                  kw: "% #{ escaped } %")
    end

    count = q.except(:select, :order, :limit, :offset).count

    versions = q.order(Arel.sql('post_versions.created_at DESC, post_versions.id DESC'))
                .limit(limit)
                .offset(offset)

    render json: { versions: serialise_versions(versions), count: }
  end

  private

  def serialise_versions rows
    user_ids = rows.map(&:created_by_user_id).compact.uniq
    users_by_id = User.where(id: user_ids).pluck(:id, :name).to_h

    rows.map do |row|
      cur_tags = split_tags(row.tags)
      prev_tags = split_tags(row.attributes['prev_tags'])

      {
        post_id: row.post_id,
        version_no: row.version_no,
        event_type: row.event_type,
        title: {
          current: row.title,
          prev: row.attributes['prev_title']
        },
        url: {
          current: row.url,
          prev: row.attributes['prev_url']
        },
        thumbnail: {
          current: nil,
          prev: nil
        },
        thumbnail_base: {
          current: row.thumbnail_base,
          prev: row.attributes['prev_thumbnail_base']
        },
        tags: build_version_tags(cur_tags, prev_tags),
        original_created_from: {
          current: row.original_created_from&.iso8601,
          prev: row.attributes['prev_original_created_from']&.iso8601
        },
        original_created_before: {
          current: row.original_created_before&.iso8601,
          prev: row.attributes['prev_original_created_before']&.iso8601
        },
        created_at: row.created_at.iso8601,
        created_by_user:
          if row.created_by_user_id
            {
              id: row.created_by_user_id,
              name: users_by_id[row.created_by_user_id]
            }
          end
      }
    end
  end

  def build_version_tags(cur_tags, prev_tags)
    (cur_tags | prev_tags).map do |name|
      type =
        if cur_tags.include?(name) && prev_tags.include?(name)
          'context'
        elsif cur_tags.include?(name)
          'added'
        else
          'removed'
        end

      {
        name:,
        type:
      }
    end
  end

  def split_tags(tags)
    tags.to_s.split(/\s+/).reject(&:blank?)
  end
end