hmm, polymorphism?

classes.rb

class DataCenter < ActiveRecord::Base
  # has configuration_type, configuration_id attributes
  def config
    configuration_type.constantize.find(configuration_id)
  end

  def config=(config)
    configuration_type = config.class.name
    configuration_id = config.id
  end
end

class AwsConfiguration < ActiveRecord::Base
end

class LinodeConfiguration < ActiveRecord::Base
end

#<DataCentre id: 1, configuration_type: 'AwsConfiguration', configuration_id: '5'>
@dc.config # => #<AwsConfiguration id: 5>

#<DataCentre id: 2, configuration_type: 'LinodeConfiguration', configuration_id: '21'>
@dc.config # => #<LinodeConfiguration id: 21>