gistfile1.rb

require 'rubygems'
require 'dm-core'
 
DataMapper.setup(:default,
  :adapter => 'mysql',
  :host => 'localhost', 
  :username => 'root',
  :database => 'dm_core_test',
  :encoding => 'utf8'
)
 
DataObjects::Mysql.logger = DataObjects::Logger.new(STDOUT, :debug)

class Entity
  include DataMapper::Resource
  property :id, Serial
  property :type, Discriminator
  property :name, String
  belongs_to :country, :nullable => true
end

class Company < Entity; end
class Person < Entity; end

class Country
  include DataMapper::Resource
  property :id, Serial
  property :name, String
  has n, :entities
end
 
DataMapper.auto_migrate!

us = Country.create(:name => "USA")
hu = Country.create(:name => "Hungary")

c = Company.create(:name => 'Whole Foods', :country => us)
p = Person.create(:name => 'John')
p = Person.create(:name => 'Fredy', :country => hu)

Entity.all.each{|e| e.country}
# Thu, 17 Sep 2009 18:50:57 GMT ~ debug ~ (0.000160) SELECT `id`, `name` FROM `countries` WHERE `id` IN (1, NULL, 2) ORDER BY `id`
# /usr/local/ruby-1.9.1-p129/lib/ruby/gems/1.9.1/gems/dm-core-0.10.0/lib/dm-core/associations/many_to_one.rb:217:in `eager_load_targets': +source+ should be Company, but was Person (ArgumentError)
# 	from /usr/local/ruby-1.9.1-p129/lib/ruby/gems/1.9.1/gems/dm-core-0.10.0/lib/dm-core/associations/relationship.rb:293:in `block in eager_load'
# 	from /usr/local/ruby-1.9.1-p129/lib/ruby/gems/1.9.1/gems/dm-core-0.10.0/lib/dm-core/associations/relationship.rb:291:in `each'
# 	from /usr/local/ruby-1.9.1-p129/lib/ruby/gems/1.9.1/gems/dm-core-0.10.0/lib/dm-core/associations/relationship.rb:291:in `eager_load'
# 	from /usr/local/ruby-1.9.1-p129/lib/ruby/gems/1.9.1/gems/dm-core-0.10.0/lib/dm-core/associations/many_to_one.rb:196:in `lazy_load'
# 	from /usr/local/ruby-1.9.1-p129/lib/ruby/gems/1.9.1/gems/dm-core-0.10.0/lib/dm-core/associations/many_to_one.rb:96:in `get'
# 	from /usr/local/ruby-1.9.1-p129/lib/ruby/gems/1.9.1/gems/dm-core-0.10.0/lib/dm-core/associations/many_to_one.rb:160:in `country'
# 	from /usr/local/ruby-1.9.1-p129/lib/ruby/gems/1.9.1/gems/extlib-0.9.13/lib/extlib/lazy_array.rb:452:in `each'
# 	from /usr/local/ruby-1.9.1-p129/lib/ruby/gems/1.9.1/gems/extlib-0.9.13/lib/extlib/lazy_array.rb:452:in `each'
# 	from relationship_2.rb:50:in `<main>'