<p>Is it the case that the entire restful verb is under a single all encompassing transaction? That is to say, if I raise a Error in the validation or callbacks at any point in the handling of a UPDATE, DELETE, or CREATE operation, is every database operation that I may have performed in previous callbacks also rolled back? Succinctly, does raising a Error on any callback or validation make it such that no change at all occurs in the database for that verb action?</p>

Some methods (create, destroy) go to the database immediately. Transactions occur by using the transaction method on classes derived from ActiveRecord as follows:
```
```
Student.transaction do
  Course.transaction do
    course.enroll(student)
    student.units += course.units
  end
end
```
```
(This example is for multiple databases. For a single database, you only need one transaction.)
You can then rollback on these transactions, and exceptions thrown within the transaction are propagated after the rollback.
This depends upon the database having transactions.
NB: save and destroy are wrapped in transactions.