code.rb

new_object = OtherDatabase::NewClass1.new
...
result = OtherDatabase::NewClass1.find(:all)

database.yml

other_database_connection:
    adapter: postgresql
    encoding: utf8
    database: database_name
    username: user
    password: password
    host: localhost

external.rb

class External < ActiveRecord::Base
  self.abstract_class = true
  establish_connection :other_database_connection
end

gistfile4.txt

model.rb

class OtherDatabase::NewClass1 < External
end