This is a sample patch for three dm gems - data_objects, do_postgres, and dm-postgres-adapter. It is not ready to be applied, since it is currently unspecced and untested. The purpose of this patch is to solicit feedback as to the architecture, to ensure

dm-postgres-adapter.rb

require 'dm-transactions/adapters/dm-do-adapter'

module DataMapper
  class Transaction

    module PostgresAdapter
      include DataObjectsAdapter

      # Produces a fresh transaction primitive for this Adapter. Fake nested
      # transactions by using save points.
      #
      # Used by Transaction to perform its various tasks.
      #
      # @return [Object]
      #   a new Object that responds to :close, :begin, :commit,
      #   and :rollback,
      #
      # @api private
      def transaction_primitive
        if current_transaction
          DataObjects::SavePoint.create_for_uri(normalized_uri, current_connection)
        else
          DataObjects::Transaction.create_for_uri(normalized_uri)
        end
      end
    end

  end
end

save_point.rb

require 'digest'
require 'digest/sha2'

# JUSTIFY: Why create a new data object type for save points?
# 
# A save point is analgous to a transaction, and a transaction has its own
# DataObject. Save points are supported by [] do databases, and [are|are
# not] part of the SQL spec.
module DataObjects

  # JUSTIFY: Why not inherit from Transaction?
  #
  # Even though they share a lot of behaviour, a SavePoint does not satisfy
  # LSP for transaction, both because it can't be used in place of a transaction
  # (save points require you to be inside a transaction), and as such because
  # of the required for the extra connection parameter.
  #
  # There is a case to be made for refactoring the common behaviour into a mixin.
  class SavePoint
    # The host name. Note, this relies on the host name being configured and resolvable using DNS
    HOST = "#{Socket::gethostbyname(Socket::gethostname)[0]}" rescue "localhost"
    @@counter = 0

    # The connection object allocated for this transaction
    attr_reader :connection
    # A unique ID for this transaction
    attr_reader :id

    # Instantiate the Transaction subclass that's appropriate for this uri scheme
    def self.create_for_uri(uri, connection)
      uri = uri.is_a?(String) ? URI::parse(uri) : uri
      DataObjects.const_get(uri.scheme.capitalize)::SavePoint.new(uri)
    end

    # JUSTIFY: Why passing in the connection? Transaction doesn't need this...
    #
    # Unlike the similar Transaction primitive, which is only created once
    # per connection and reused for each transaction, this primitive will
    # be created once for each save point, since each save point requires
    # its own unique identify. As a result, the extra connection paramater
    # is required to allow the existing connection to be reused.
    def initialize(uri, connection)
      @connection = connection
      @id = Digest::SHA256.hexdigest("#{HOST}:#{$$}:#{Time.now.to_f}:#{@@counter += 1}")
    end

    def begin
      run %{SAVEPOINT "#{@id}"}
    end

    def commit
      run %{RELEASE SAVEPOINT "#{@id}"}
    end

    def rollback
      run %{ROLLBACK TO SAVEPOINT "#{@id}"}
    end

    private

    # JUSTIFY: Transaction doesn't have this extract method refactoring, why here?
    #
    # Because I see no reason for this not also to be applied to Transaction
    def run(cmd)
      connection.create_command(cmd).execute_non_query
    end
  end

end

This has been effectively merged in:

• datamapper/do@74645c4
• datamapper/dm-transactions@242e604