Gemfile

source "https://rubygems.org"

gem 'activerecord'
gem 'ar-octopus', require: 'octopus'
gem 'sqlite3'

reproduce.rb

#!/usr/bin/env ruby

require "octopus"
class User < ActiveRecord::Base
  has_many :works
end
class Work < ActiveRecord::Base

end

ActiveRecord::Base.establish_connection YAML.load_file("config/database.yml")["development"]
ActiveRecord::Base.logger = Logger.new(STDOUT)
ActiveRecord::Base.connection.execute("create table if not exists user (id int); create table if not exists works (id int); create table if not exists user_works (user_id int, work_id int);")

a = User.using(:read_slave).first
a.works
b = User.last
b.save