adapter_shared_spec.rb

share_examples_for "An Adapter" do

  def adapter_supports?(method)
    @adapter.class.instance_methods(false).include?(method.to_s)
  end

  before :all do
    raise "+@adapter+ should be defined in before block" unless instance_variable_get("@adapter")

    # More setup ...
  end

  if adapter_supports?(:create)
    describe '#create' do
      before do
        @adapter.create(:foo, :bar)
        # ...
      end

      it 'should do something' do
        # ...
      end

      it 'should do something else' do
        # ...
      end
    end
  end

end