Add records to has_many :through using <<, push, and concat by creating the association record. Raise if base or associate are new records since both ids are required to create the association. #build raises since you can't associate an unsaved record. #create! takes an attributes hash and creates the associated record and its association in a transaction.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@4786 5ecf4fe2-1ee6-0310-87b1-e25e094e27de