I have a relation A->B. one-to-one.

I want to delete both the objects as part of a transaction. Do I need to
worry about in what order I need to delete these objects.

Can I delete B first , then A and then commit transaction or do I need to
worry about constraint voilations when deleting
( eg. I would delete A first and then delete B)

While inside the transaction context do I need to worry about constraints
and voilations.

You don't have to worry about that.

Actually you *do*, if you don't have cascade delete enabled ;-)

So enable cascade delete and you will be fine.

P.S. Note that my comment applies to one-to-ones only. They are a bit special. In the case of many-to-ones, christian is correct.