database.yml

development:
  adapter: master_slave
  master:
    adapter: mysql
    database: master
    host: master_server
  slave:
    adapter: mysql
    database: slave
    host: localhost

master_slave_adapter.rb

require 'forwardable'

module DataMapper::Adapters

  class MasterSlaveAdapter < AbstractAdapter
    extend Forwardable

    attr_reader :name

    def_delegators :@master, :create, :update, :delete
    def_delegators :@slave, :read

  private

    def initialize(name, options)
      super(name, options)

      assert_kind_of 'options', @options[:master], Hash
      assert_kind_of 'options', @options[:slave],  Hash

      @master = ::DataMapper.setup("#{name}_master".intern, @options[:master])
      @slave  = ::DataMapper.setup("#{name}_slave".intern,  @options[:slave] )
    end

    def method_missing(meth, *args, &block)
      @slave.send(meth, *args, &block)
    end

  end

end