# project/comment.rb
module Project
  class Comment

    include DataMapper::Resource
    include DataMapper::AutoMigrations

    property :body, DataMapper::Types::Text

    belongs_to :post

  end
end

# project/post.rb
require 'project/comment'

module Project
  class Post

    include DataMapper::Resource
    include DataMapper::AutoMigrations

    property :title, String

    property :body, DataMapper::Types::Text

    # should reference Project::Comment since were in the Project name-space
    has n, :comments

  end
end

irb> require 'data_mapper'
irb> DataMapper.setup(:default, "sqlite3:memory:")
irb> require 'project/comment'
irb> require 'project/post'
irb> DataMapper.auto_migrate!
NameError: uninitialized constant Comment
	from /usr/lib64/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/support/object.rb:16:in `const_get'
	from /usr/lib64/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/support/object.rb:16
	from /usr/lib64/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/support/object.rb:15:in `each'
	from /usr/lib64/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/support/object.rb:15
	from /usr/lib64/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/support/object.rb:9:in `call'
	from /usr/lib64/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/support/object.rb:9:in `default'
	from /usr/lib64/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/support/object.rb:9:in `[]'
	from /usr/lib64/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/support/object.rb:9:in `find_const'