Using standalone ActiveRecord is really easy

standalone_active_record.rb

require "active_record"
require "pg"

# AR connection
ActiveRecord::Base.establish_connection(
  adapter:  "postgresql",
  database: "lynda_test",
  username: "<db_username>",
  password: "<db_password>",
  host:     "localhost",
  port:     "5432",
)

# models (tables have to exist in the DB)
class Customer < ActiveRecord::Base
  has_many :sales
end

class Item < ActiveRecord::Base
  has_many :sales
end

class Sale < ActiveRecord::Base
  belongs_to :customer
  belongs_to :item
end