Rails - May 2006 - Noob: Validation Errs Don''t Display When Saving Object Tree

I''ve got the following model:

User < AR::Base
  has_one :user_detail

UserDetail < AR::Base
  belongs_to :user
  has_one :address

Address < AR::Base
  belongs_to :user_detail

I''ve a form to gather the data, and it includes partials for each
class.
Each partial has an error_messages_for ''class'' snippet which
should
display the errors relevant to each class.
On submission I''m doing:

@user = User.new(params[:user])
@user.user_detail = UserDetail.new(params[:user_detail])
@user.user_detail.address = Address.new(params[:address])

If I complete the form, all is well and the object tree gets saved 
correctly to the DB. However, if I deliberately omit key information 
from the form, only errors relevant to the User class are displayed. The 
User.save succeeds even if UserDetail or Address validations fail.

I thought that a save would fail if any of the objects on the model tree 
failed validation, and that all saves are transactional, so if one 
element failed the whole object tree got rolled back?

How can I ensure that
a) a validation failure in either UserDetail or Address causes a 
rollback of the save
b) validation errors in UserDetail and/or Address get displayed in the 
form?

thanks,
donncha

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