I have many-to-one uni-directional mapping between class A and class B. The relationship specified in the mapping file for class B as follows:

<many-to-one
name="A"
class="pakage.A"
column="A_ID"
/>


I able to delete class B using session.delete(object of B). But I could not use this method to delete the object of A when it's refered by object B. I am getting error message for foreign constraint violation.
Do I have to use queries to set the foreign key to null, before deleting? Or Are there parameter(attributes) exist to do this?

Make it bi-directional with correct 'inverse' attribute.

That's right, but I want to maintain the relationship, uni-directional. Since if the relationship is changed, I have to chage a lot in my code.