Rails: how to extend a gem's ActiveRecord child class?

Im having problems extending a class which is defined in a gem and is a child of ActiveRecord::Base.

The only thing i'd like to extend this class with is: has_many :promos

The extending however tends to rule out the original class. The errors i'm getting:

PGError: ERROR:  relation "sites" does not exist
LINE 4:              WHERE a.attrelid = '"sites"'::regclass
                                        ^
:             SELECT a.attname, format_type(a.atttypid, a.atttypmod), d.adsrc, a.attnotnull
              FROM pg_attribute a LEFT JOIN pg_attrdef d
                ON a.attrelid = d.adrelid AND a.attnum = d.adnum
             WHERE a.attrelid = '"sites"'::regclass
               AND a.attnum > 0 AND NOT a.attisdropped
             ORDER BY a.attnum

Checking the class in the console gives:

Cms::Site(Table doesn't exist)

The original class has this method which probably isn't invoked anymore:

set_table_name :cms_sites

Btw. i'm trying to extend the Site class from the comfortable_mexican_sofa plugin.

This is the file which should extend the class:

# lib/comfortable_media_sofa/comfortable_media_sofa.rb
require 'comfortable_mexican_sofa'

module Cms
  class Site < ActiveRecord::Base
    has_many :promos
  end
end

Which gets loaded here:

require File.expand_path('../boot', __FILE__)

require 'rails/all'

Bundler.require(:default, Rails.env) if defined?(Bundler)

module Mkturbo
  class Application < Rails::Application
    config.autoload_paths += %W(#{config.root}/vendor/gems/comfortable_mexican_sofa-0.0.18)
    config.autoload_paths += %W(#{config.root}/lib/comfortable_media_sofa)
    config.plugins = [ :comfortable_mexican_sofa, :comfortable_media_sofa, :all ]

    # ....
  end
end

And is required in the top of the comfortable_mexican_sofa initializer:

# config/initializers/comfortable_mexican_sofa.rb
require 'comfortable_media_sofa'

How can i do this? Is a requirement order issue or am i extending it the wrong way? Many thanks in advance!