hatbm relationship can edit, but not create.

Hey all,

Following this Railscast (http://railscasts.com/episodes/17-habtm-
checkboxes) I have a set up a form with checkboxes that work great for
updating records in a hatbm relationship.

But it only works if I'm doing an update to an existing record. When I
try to create, I get a validation error.

In my app I have Sessions which can have many Learners. In the Session
edit/create form there's a list of all the Learners with check boxes
for selecting them.

I can edit a Session and check/uncheck Learners and the changes are
saved. When I create a new session, I get a "Learners is invalid"
validation error.

This is probably happening because the Learner ids can't be saved into
the Learners_Sessions table without a session id, and the the session
id isn't created until save. So when I do this:

def create
@session = @user.project.sessions.new(params[:session])
if @session.save

it throws out the validation error.

Any ideas on how I can work around this? I could create a bare-bones
Session object, save it and then update it with the full params, but
that seems inelegant. I figure there has to be a better solution, I
just don't know what it is.