Schema support for your adapter

schema.rb

require 'dry/types'

module ROM
  module Plugins
    module Relation
      module Schema
        Undefined = Object.new.freeze
        
        def self.included(klass)
          super
          klass.extend(ClassMethods)
          klass.include(InstanceMethods)
        end

        module InstanceMethods
          def schema
            self.class.schema
          end

          def each
            return to_enum unless block_given?
            super { |e| yield(schema.call(e)) }
            self
          end

          def attributes
            schema.member_types.keys
          end

          def base_name
            dataset.table.name
          end
        end

        module ClassMethods
          def schema(value = Undefined)
            if value != Undefined
              @__schema = Dry::Types['hash'].symbolized(value)
            elsif !schema?
              @__schema = NullSchema.new(self.to_s)
            end
            @__schema
          end

          def schema?
            !@__schema.nil?
          end
        end


        class NullSchema
          def initialize(relation_name)
            @relation_name = relation_name
          end

          def call(value)
            value
          end

          def member_types
            raise("no schema defined for #{@relation_name}")
          end
        end
      end
    end
  end
end

ROM.plugins do
  register :schema, ROM::Plugins::Relation::Schema, type: :relation
end