Fiddling with the shockingly documented AREL library to produce a join using an aggregate SQL function (max) in the hopes I'd be able to chain it. ALAS! Only after 2 hours did I realize this ain't gonna works yo.

checklist.rb

class Checklist < ActiveRecord::Base
  scope :current, -> {
    c = Arel::Table.new(:checklists, as: 'c')
    r = Arel::Table.new(:checklists, as: 'r')
    sql = c.project(Arel.star)
      .join(r.project(r[:version_id], r[:version_no].maximum.as('vno'))
        .group(r[:version_id]).as('r'))
      .on(r[:version_id].eq(c[:version_id])
      .and(r[:vno].eq(c[:version_no])))
    find_by_sql(sql)
  }
  
  # which is equivalent to:
  #  find_by_sql("
  #    SELECT c.*
  #    FROM checklists c
  #    INNER JOIN (
  #      SELECT version_id, MAX(version_no) as vno
  #      FROM checklists
  #      GROUP BY version_id
  #    ) r
  #    ON r.version_id = c.version_id
  #    AND r.vno = c.version_no
  #  ")
  #
  # or:
  #   SELECT c.*
  #   FROM checklists c
  #   WHERE version_no = (
  #     SELECT version_no 
  #     FROM checklists c2
  #     WHERE c2.version_id = c.version_id
  #     ORDER BY c2.version_no desc
  #     LIMIT 1
  #  )
  #
  # Don't ask me which is faster, I don't know.

end

schema.rb

# ...
  create_table "checklists", force: :cascade do |t|
    t.string   "name",           limit: 255,                               null: false
    t.text     "description",    limit: 65535
    t.integer  "creator_id",     limit: 4
    t.string   "type",           limit: 255,   default: "CustomChecklist", null: false
    t.integer  "version_id",     limit: 4
    t.integer  "version_no",     limit: 4,                                 null: false
    t.integer  "duration",       limit: 4
    t.text     "meta_data",      limit: 65535
    t.datetime "schedule_start"
    t.datetime "schedule_end"
    t.text     "schedule",       limit: 65535
    t.datetime "deleted_at"
    t.datetime "created_at",                                               null: false
    t.datetime "updated_at",                                               null: false
  end
# ...

Reminder to self to thank Jiří Pospíšil (http://jpospisil.com/2014/06/16/the-definitive-guide-to-arel-the-sql-manager-for-ruby.html)