establish_connection.rb

require 'active_record'

ActiveRecord::Base.establish_connection({
    adapter: 'postgres', # or mysql2
    host: 'localhost',
    username: 'username',
    password: 'password',
    database: 'dbname',
})
# ActiveRecord::Base.establish_connection("postgres://username:password@host/dbname")

class User < ActiveRecord::Base
  self.table_name = "my_users"
  self.primary_key = "id"
end