adapter_spec.rb

describe MyCustomerAdapter do
  should_support_adapter_interface :create
  should_support_adapter_interface :delete
end

macro.rb

module SpecHelpers
  module AdapterMethods
     def should_support_adapter_interface(interface)
       if described_type.instance_methods.include?(interface.to_s)
         # split out your interfaces in their own shared specs
         it_should_behave_like "an adapter supporting the :#{interface} method"
       else
         it "needs to support the :#{interface} method"
       end
     end
  end
end

spec_helper.rb

Spec::Runner.configure do |config|
  config.extend SpecHelpers::AdapterMethods
end