dm-bug.rb

require 'rubygems'
require 'dm-core'
 
class Mother
  include DataMapper::Resource
  
  property :id,               Serial
  
  has 1, :baby
  has 1, :child
 
end

class Baby
  include DataMapper::Resource
  
  property :id,   Serial
  property :name, String
  property :mother_id, Integer, :required => false # avoiding some IntegrityErrors
  property :type,             Discriminator
end

class Child < Baby
end
 
 
#DataMapper::Logger.new(STDOUT)
DataMapper.setup(:default, "sqlite3::memory:")
DataMapper.auto_migrate!
 
e = Mother.create
e.baby = Baby.create( :name => "One" )
e.child = Child.create( :name => "Two" )
e.save

puts "Works, the associations is two separate objects like expected."
p e
p e.baby
p e.child

e = Mother.create
#e.baby = Baby.create( :name => "One" )
e.child = Child.create( :name => "Two" )
e.save

puts "Fails, both associations is the same object even though only one is set."
p e
p e.baby
p e.child # i'd expect this to be nil