Adds methods to Rails has_many and HABTM associations that speed up common operations:

a. fast_copy
A method called fast_copy is added to has_and_belongs_to_many association collections that makes the process of cloning HABTM associations MUCH more efficient. Simply replace person1.items = person2.items with person1.items.fast_copy(person2) and your database, network and RAM will thank you. See below for more details.

b. ids
A method called ids is added to has_many and has_and_belongs_to_many association collections. It returns the list of object ids in the association collection without unnecessarily instantiating the objects.

Read the full announcement for more details:
http://blog.zvents.com/2006/11/3/rails-plugin-association-collection-tools