general troubleshooting

I want to do something like:

raise Tag.find_or_create_by_tag([:which_tag])

to troubleshoot the add_some_tags method of the feeds controller:

http://strawr.googlecode.com/svn/trunk/app/controllers/feeds_controller.rb

How would I go about that? the error message I'm troubleshooting:

ActiveRecord::HasManyThroughCantAssociateNewRecords in
FeedsController#add_some_tags

Cannot associate new records through 'Feed#colors' on '#'. Both records
must have an id in order to create the has_many :through record
associating them.

thanks,

Thufir