gistfile1.rb

DataMapper.setup(:default, :adapter => 'mysql', :database => "test", :username => "root")
DataMapper.setup(:dual, :adapter => "dual", :default => repository(:default), :redis => {:db => 1})

   class DualAdapter < AbstractAdapter
      def create(query)
        @default.create query
      end

      def read(query)
        query = simplify(query) if simplify_applies?(query)
        @default.read query
      end

      def simplify(query)
        equal_to = query.conditions.operands.select { |o|
          o.is_a?(DataMapper::Query::Conditions::EqualToComparison) &&
          o.subject.parent_model == User &&
          o.subject.child_model == Status
        }
          
        equal_to.each { |e| query.conditions.operands.delete e }
        raise "dupa" if equal_to.size > 1
        equal_to = equal_to.first

        child_model = equal_to.subject.child_model
        parent_key = equal_to.subject.parent_key.first.name

        key = redis_key(child_model, equal_to.value.send(parent_key))
        child_ids = @redis.zrevrange(key, query.offset, query.limit - 1) #TODO use either zrevrange or zrange depending on query.order?

        query.conditions << DataMapper::Query::Conditions::InclusionComparison.new(Status.properties[:id], child_ids)
        query
      end

      def redis_key(model, key)
        #statuses ids are stored in Redis zsets
        "#{model.name.split("::").last.downcase.first}:#{key}"
      end

      def simplify_applies?(query)
        query.conditions.operands.any? { |o| 
          o.is_a?(DataMapper::Query::Conditions::EqualToComparison) &&
          o.subject.parent_model == User &&
          o.subject.child_model == Status
        }
      end

      def initialize(name, uri_or_options)
        super
        @default = @options[:default]
        @redis = Redis.new(@options[:redis])
      end
    end

class Status
  include ::DataMapper::Resource

  def self.default_repository_name; :dual; end

  property :id, Serial
  property :user_id, Integer

  belongs_to :user
end

class User
  include DataMapper::Resource
     
  property :id, Serial
  property :login, String

  has n, :statuses, :repository => Status.default_repository_name, :order => [:id.desc]
end

User.first.statuses(:limit => 5) #  superb fast SELECT `id`, `user_id` FROM `statuses` WHERE `id` IN (190, 189, 188, 187, 186) ORDER BY `id` DESC LIMIT 5