This issue http://stackoverflow.com/questions/8899272/rails-3-undefined-method-zero-for-nilnilclass-in-many-to-many-relationsh http://stackoverflow.com/questions/9289535/rails-3-2-uniqueness-validation-raises-undefined-method-zero-for-nilnilclass also seems to affect this gem when using this within Rails 3. Is there a workaround?