i have a student object and student has set of phone numbers which is stored in student_phone table.
i have given the mapping as
<set name="studentPhoneNumbers" table="STUDENT_PHONE" cascade="delete-orphan">
<key column="STUDENT_ID" />
<many-to-many column="PHONE_ID" unique="true"
class="com.test.tutorials.Phone" />
</set>
i have loaded the student object and i have given session.setReadOnly(student,false);
after loading the student i have done student.getStudentPhoneNumbers().removeAll(Collection....);
after that we are updating the student name and calls session.update(student).

when it issues the update , the following queries are issued.
Hibernate: update Student set STUDENT_NAME=?, STUDENT_ADDRESS=? where STUDENT_ID=?
Hibernate: delete from STUDENT_PHONE where STUDENT_ID=?
Hibernate: delete from PHONE where PHONE_ID=?

My doubt is why it issues Delete to Phone table.
is session.setReadOnly(student,true); why it dosent do and update to student table. only the following query is issued
delete from STUDENT_PHONE where STUDENT_ID=?

can anyone suggest the reason