I have below class structure,

@Entity
@Table(name = "A")
class A
{
@OneToMany(mappedBy = "aVal")
List<B> bValues;
}

@Entity
@Table(name = "B")
class B
{
@Transient
List<C> bValues;
}

@Entity
@Table(name = "C")
class C
{

}

I do not want C table to be updated when entity of type A is saved.
However, when I do session.save(a) an update is getting fired for entity C which throws org.hibernate.StaleStateException

Please can anybody elaborate what I am doing wrong here.

Where is your @Id annotation? Did you always annotate fields, not getters?

I have not completely described the code. There is a @Id annotation in the class I am using.
I tried using the @Transient annotation on both/on field/on getter none of them worked.

Well, then you need to provide more information so others can help you. Like the code inside the update-transaction and maybe the sql-statements that get submitted also.

I managed to resolve it.
I am using Spring MVC along with hibernate. The hibernate enity is being used as a Spring MVC form backing object.
A Spring SimpleFormController has two callback methods formBackingObject() and onSubmit(). (formBackingObject is called every time before onSubmit() if your form has sessionForm=false)

Earlier I was deleting/saving the C values in database in formBackingObject() callback and then saving A values in onSubmit(). Somehow the C values changed in formBackingObject() was not reflecting in the form backing object (I am using A object).

I managed to fix it by moving the C values deleting/saving code to onSubmit(). But still I am not sure why session.save(A) was calling update on C.