DataMapper InMemoryAddapter that respects your keys.

gistfile1.rb

module DataMapper
  module Adapters
    # DataMapper.setup(:default, 'in_memory_with_keys::')
    class InMemoryWithKeysAdapter < InMemoryAdapter
      def create(resources)
        records = records_for(resources.first.model)

        resources.each do |resource|
          key_names = resource.model.properties(self.name).select(&:key?).map(&:name).map(&:to_s)

          record_as_fields = attributes_as_fields(resource.attributes(nil))
          keys = extract_key_parts(key_names, record_as_fields)

          collision = records.any? do |record|
            extract_key_parts(key_names, record) == keys
          end

          raise 'DUPLICATE!' if collision

          initialize_serial(resource, records.size.succ)
          records << record_as_fields
        end
      end

      private

      def extract_key_parts(key_names, hash)
        hash.values_at(*key_names)
      end
    end
    const_added(:InMemoryWithKeysAdapter)
  end
end