user.rb

class User < Ohm::Model

  set :keywords, String

end
------------------------------------
ruby-1.9.2-p136 :043 > u=User.create
 => #<User:1 keywords=#<Set (String): []>> 
ruby-1.9.2-p136 :044 > u.keywords << 'hi'
NoMethodError: undefined method `id' for "hi":String
	from /Users/Chris/.rvm/gems/ruby-1.9.2-p136@rails3/bundler/gems/ohm-8682aa6de4da/lib/ohm.rb:513:in `<<'
	from (irb):44
	from /Users/Chris/.rvm/gems/ruby-1.9.2-p136@rails3/gems/railties-3.0.3/lib/rails/commands/console.rb:44:in `start'
	from /Users/Chris/.rvm/gems/ruby-1.9.2-p136@rails3/gems/railties-3.0.3/lib/rails/commands/console.rb:8:in `start'
	from /Users/Chris/.rvm/gems/ruby-1.9.2-p136@rails3/gems/railties-3.0.3/lib/rails/commands.rb:23:in `<top (required)>'
	from script/rails:6:in `require'
	from script/rails:6:in `<main>'
ruby-1.9.2-p136 :045 >