Before putting out my problem these are the various imp things.

Using:
Hibernate 2.1
Database:DB2
Application - Web App -- Entities are loaded in 1 session and then saved in another session.

Now the problem Description------

EntityA has a relationship with EntityB, EntityC where EntityA will always be a child of EntityB and can also be a child of EntityC based on some business rule but not mandatory.

Entity A mapping document(relationship) looks like this---

<many-to-one name="EntityB" class="test.EntityB" not-null="true" >
<column name="codea" />
</many-to-one>

<many-to-one name="EntityC" class="test.EntityC" not-null="true" >
<column name="forcode" />
</many-to-one>
------------------------------------------------------
So for EntityA to exist "EntityB" must be present but not necessarily EntityC(only when required).

Now when i am trying to save "EntityA" (without an instance of EntityC which is not needed always) there comes this error
--------------------------------------------
net.sf.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: test.EntityC
----------------------------------------------

Could you guide me where i am going wrong.

Thanks

In the object graph you saved, EntityA instance refers to an EntityC instance which is not attach to the session.
Remove not-null="true" and don't setEntityC() in EntityA.