spec_extensions.rb

$_groups = []

class Spec::Runner::Configuration
  def before_spec
    before(:all) do
      top_level_group = self.class.to_s[/Merb::Test::ExampleGroup::([^:]+)/, 1]
      unless $_groups.any? { |g| top_level_group == g }
        $_groups << top_level_group
        yield
      end
    end
  end
end

spec_helper.rb

Spec::Runner.configure do |config|
  config.include(Merb::Test::ViewHelper)
  config.include(Merb::Test::RouteHelper)
  config.include(Merb::Test::ControllerHelper)

  config.before_spec do
    DataMapper.auto_migrate!
  end
  ...
end