More Advanced Database Features and Rails

Occasionally, you will have the need to connect to several different databases from one application. This is useful for migrating from an old schema to a new one. It is also helpful if you have differing data requirements within one application; perhaps some data is more critical and is stored on a high-availability database cluster. In any case, it is easy in Rails. First, specify multiple database environments in the database.yml configuration file:

  legacy:
    adapter: mysql
    database: my_db
    username: user
    password: pass
    host: legacy_host

  new:
    adapter: mysql
    database: my_db
    username: user
    password: pass
    host: new_host

Then, you can simply refer to these configuration blocks from the ActiveRecord class definition using the ActiveRecord::Base.establish_connection method:

  class LegacyClient < ActiveRecord::Base
    establish_connection "legacy"
  end

  class Client < ActiveRecord::Base
    establish_connection "new"
  end

This approach also works with multiple Rails environments. Just specify each environment in the database.yml file as usual:

  legacy_development:
    # ...

  legacy_test:
    # ...

  legacy_production:
    # ...

  new_development:
    # ...

  new_test:
    # ...

  new_production:
    # ...

Then, use the RAILS_ENV constant in the database configuration block name:

  class LegacyClient < ActiveRecord::Base
    stablish_connection "legacy_#{RAILS_ENV}"
  end

  class Client < ActiveRecord::Base
    establish_connection "new_#{RAILS_ENV}"
  end

You can go one step further and DRY this code up by using class inheritance to define which database an ActiveRecord class belongs to:

  class LegacyDb < ActiveRecord::Base
    self.abstract_class = true
    establish_connection "legacy_#{RAILS_ENV}"
  end

  class NewDb < ActiveRecord::Base
    self.abstract_class = true
    establish_connection "new_#{RAILS_ENV}"
  end

  class LegacyClient < LegacyDb
  end

  class Client < NewDb
  end

The self.abstract_class = true statements tell ActiveRecord that the LegacyDb and NewDb classes cannot be instantiated themselves; since they represent database connections, they are not backed by concrete tables in the database.