gistfile1.rb

require 'rubygems' 
require 'dm-core'
require "dm-migrations"
require 'dm-sqlite-adapter'

DataMapper.setup(:default, 'sqlite::memory:')
DataMapper.auto_upgrade!

class Event 
    include DataMapper::Resource 

    property :id,   Serial 
    property :signature, Integer

    belongs_to :signature, :parent_key => [:sig_id], :child_key => [:signature]
end

class Signature
  include DataMapper::Resource

  property :sig_id, Serial

  has n, :events, :child_key => [:signature]
end

DataMapper.finalize

gistfile2.sql

 ~ (0.000047) SELECT sqlite_version(*)
 ~ (0.000086) DROP TABLE IF EXISTS "events"
 ~ (0.000011) PRAGMA table_info("events")
 ~ (0.000231) CREATE TABLE "events" ("id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, "signature" INTEGER)
 ~ (0.000030) DROP TABLE IF EXISTS "signatures"
 ~ (0.000008) PRAGMA table_info("signatures")
 ~ (0.000087) CREATE TABLE "signatures" ("sig_id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT)