preventing foreign key to set to null

It seems that if set "auto-retrieve" to false for foreign key object, when I 
call broker.store(), it will also set the foreign key to null also!

Supposing I have 2 tables Checks and CheckBooks

<class-descriptor
        class="Check"
        table="Checks">
     <field-descriptor
            name="checkBookId"
            column="CheckBookId"
            jdbc-type="BIGINT"
            />
     ....
     <reference-descriptor
            name="checkBook"
            class-ref="CheckBook"
            auto-retrieve="false"
            auto-update="false"
            auto-delete="false">
        <foreignkey field-ref="checkBookId"/>
    </reference-descriptor>
</class-descriptor>

As I call
broker.retrieveObject,
the checkbook attribute is null because auto-retrieve = false

As I call
broker.store(check);
OJB will set checkBookId to null also!!

Is the only way to do is to use proxy??
Is there any other easy way to do prevent this from happening?