how to understand callback and validate_associated

1 after_update callback is wrapped in transaction means:no matter
exception occurs during saving an existing record, or in the
after_update method body,both the existing record and record
manipulation in the after_update body are all rollbacked?

2 class Book < ActiveRecord::Base
    has_many :pages
    validates_associated :pages
    validates_presence_of :name
   end

validate_associated means: when i invoke book.save to save book and it's
associations,no matter invalidation occurs during saving book self or
saving any association,both book and all the associations don't be
saved?

I don't understand these from document.can you help me to make it sure?
Thanks!

anyone help me?

hi
  who know about it? thanks!

hi
who know about it? thanks!

Maybe you could rephrase your question - I for one couldn't understand
what you were asking

Fred

thank!
I copy the after_update() doc as following:

I have described the problem clearly.Can you help me?

Correct. The entirety of the save process (validations, before_save,
after_save etc.) is wrapped in a single transaction.

Fred

Thanks,fred.