Datamapper DataMapper

I don't really know how to describe this bug, except in code:

require 'rubygems'
require 'dm-core'
require 'dm-migrations'
require 'dm-constraints'

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, 'postgres://localhost/test') # createdb test

class Post
  include DataMapper::Resource

  property :id, Serial
  belongs_to :author
end

class User
  include DataMapper::Resource

  property :id, Serial
end

class Author
  include DataMapper::Resource

  belongs_to :user, :key => true, :child_key => [:id]

  has n, :posts
end


DataMapper.finalize
DataMapper.auto_migrate!

Fails with:

 ~ (0.000255) ALTER TABLE "posts" ADD CONSTRAINT "posts_author_fk" FOREIGN KEY () REFERENCES "authors" () ON DELETE NO ACTION ON UPDATE NO ACTION
 ~ ERROR:  syntax error at or near ")"
LINE 1: ...ts" ADD CONSTRAINT "posts_author_fk" FOREIGN KEY () REFERENC...
                                                             ^
 (code: 16801924, sql state: 42601, query: ALTER TABLE "posts" ADD CONSTRAINT "posts_author_fk" FOREIGN KEY () REFERENCES "authors" () ON DELETE NO ACTION ON UPDATE NO ACTION, uri: postgres://localhost/test)
/Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-do-adapter-1.0.2/lib/dm-do-adapter/adapter.rb:63:in `execute_non_query': ERROR:  syntax error at or near ")" (DataObjects::SyntaxError)
LINE 1: ...ts" ADD CONSTRAINT "posts_author_fk" FOREIGN KEY () REFERENC...
                                                             ^
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-do-adapter-1.0.2/lib/dm-do-adapter/adapter.rb:63:in `block in execute'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-do-adapter-1.0.2/lib/dm-do-adapter/adapter.rb:260:in `with_connection'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-do-adapter-1.0.2/lib/dm-do-adapter/adapter.rb:61:in `execute'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-constraints-1.0.2/lib/dm-constraints/adapters/dm-do-adapter.rb:64:in `create_relationship_constraint'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-constraints-1.0.2/lib/dm-constraints/migrations.rb:51:in `block in execute_each_relationship'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-constraints-1.0.2/lib/dm-constraints/migrations.rb:50:in `each_value'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-constraints-1.0.2/lib/dm-constraints/migrations.rb:50:in `execute_each_relationship'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-constraints-1.0.2/lib/dm-constraints/migrations.rb:43:in `auto_migrate_up_constraints!'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-migrations-1.0.2/lib/dm-migrations/auto_migration.rb:45:in `block in repository_execute'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-core-1.0.2/lib/dm-core/support/descendant_set.rb:68:in `block in each'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-core-1.0.2/lib/dm-core/support/descendant_set.rb:67:in `each'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-core-1.0.2/lib/dm-core/support/descendant_set.rb:67:in `each'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-migrations-1.0.2/lib/dm-migrations/auto_migration.rb:44:in `repository_execute'
    from /Users/xavier/.rvm/gems/ruby-1.9.2-p0/gems/dm-constraints-1.0.2/lib/dm-constraints/migrations.rb:12:in `auto_migrate!'
    from dm-load-order.rb:32:in `<main>'

The work around is to move the Post definition below the Author definition (or if using autoloading and rails, add a reference to Author at the top of the post.rb file to force it to be loaded before the rest of the file is run)