Save's implicit transactions when updating?



When saving a new model object that has child objects (corresponding
to a child table), activerecord cleverly does the save as an implicit
transaction and saves the model object and the children objects.

However, after reading the model object from the database and updating
it and it’s children, activerecord does not do an implict transaction
when saving the model object. The modified object does get saved but
the modified children do not get saved. Why not? Major pain for my
current application.