associating has_many through on create

when my models had a HABTM relationship, i got the cool method that
assigns relational items when creating a new record. the simple
example that i can think of would be a habtm between posts and
categories.

if i'm creating a new post, i can just use the category_ids= method
and everything works. now that i'm using HMT, i don't have that nice
method anymore. what's the best way of associating categories to posts
on create.