Extending acts_as_taggable_on_steroids

I'm extending acts_as_taggable_on_steroids to allow me to give a value
to each tag.

I've got a User table, a Tag table, and a Value table. I want each
Value to refer to a specific tag and a specific user.

Like so:

class Value < ActiveRecord::Base
  belongs_to :tag
  belongs_to :user
end

When you're viewing a user, you can see its list of tags, and click on
one to add a new value to the tag.

What's the best way of properly assigning the user_id and the tag_id
to the new Value object?

Should I pass them in URL params, put them in the session? Is there a
more elegant solution? What am I missing?

Thanks,

Jeff Coleman