Advanced Rails - Building Industrial-Strength Web Apps in Record Time

118 | Chapter 4: Database

You can go one step further and DRY this code up by using class inheritance to
define which database anActiveRecord 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

Theself.abstract_class = truestatements tell ActiveRecord that theLegacyDband
NewDbclasses cannot be instantiated themselves; since they represent database con-
nections, they are not backed by concrete tables in the database.

Magic Multi-Connections

Dr Nic Williams’s Magic Multi-Connections gem (http://magicmodels.rubyforge.org/
magic_multi_connections/) allows you to connect to different databases concurrently
from the same application. This is very useful when using one master and several
read-only slaves serving the same models. The syntax is transparent; it uses module
namespaces and imports the models (ActiveRecord::Base subclasses) into the
namespaces.

For a single-master situation, you could define another database connection in
database.yml for the read slave:

read_slave:

adapter: postgresql

database: read_only_production

username: user

password: pass

host: read_slave_host

This database is backed by a module, which mirrors the ActiveRecord classes using
this database connection:

require 'magic_multi_connections'

module ReadSlave

establish_connection :read_slave

end

Now, all pre-existing models can be accessed through theread_slaveconnection by
prefixing the model class withReadSlave::.