adapter_helpers.rb

    def supported_by(*adapters)
      adapters = get_adapters(*adapters)

      PRIMARY.only(*adapters).each do |adapter, connection_uri|
        # keep track of the current adapters
        AdapterHelpers.current_adapters << adapters

        message = "with #{adapter}" if adapters.length > 1

        with_adapter_spec_wrapper(message) do

          before(:each) do
            # store these in instance vars for the shared adapter specs
            @adapter = DataMapper.setup(:default, connection_uri)
            @repository = repository(:default)

            begin
              DataMapper.auto_migrate!
            rescue NotImplementedError
              # do nothing when not supported
            end
          end

          yield adapter
        end

        AdapterHelpers.current_adapters.pop
      end
    end

adapter_shared_spec.rb

  describe "#read_one" do
    before do
      @return = @adapter.read_one(DataMapper::Query.new(@repository, @model, :id => 1))
    end
    
    it "should return a DataMapper::Resource" do
      @return.should be_a_kind_of(@model)
    end
    
    it "should return nil when no resource was found" do
      @adapter.read_one(DataMapper::Query.new(@repository, @model, :id => nil)).should be_nil
    end
  end