require 'active_record'
# Rails で言うところの config/database.yml
ActiveRecord::Base.establish_connection(
    adapter:  'mysql2',
    host: 'localhost',
    username: 'username',
    password: 'password',
    database: 'database',
    # ....
)

# 使いたいテーブルのモデル
class User < ActiveRecord::Base
  # relation とか
end

# ...