This is a typical scenario where you select has_many through over habtm. As a rule, if you only need to associate two models, no other info needed to be stored in the association, use habtm. For most cases, you have to use has_many through. You case falls under this scenario.
You want to save the hourly rate in the table that associates a Profession
and a Company
. If you have existing data that you want to migrate, you may want to look at this post How to migrate has_and_belongs_to_many to has_many through?. If you can drop the joins table you use for the habtm association, just drop it and create a new table.