I am learning Hibernate. I have a bidirectional many-to-one relation between Groups and Subgroups. Group is the inverse side. I am using XML mappings:
Code:
<class name="Group" table="`group`">
<id name="id"><generator class="native"/></id>
<set name="subgroups" inverse="true" cascade="save-update">
<key column="group_id" not-null="true"/>
<one-to-many class="Subgroup"/>
</set>
</class>
<class name="Subgroup" table="`subgroup`">
<id name="id"><generator class="native"/></id>
<many-to-one name="group" column="group_id" not-null="true"/>
</class>
I am using a set. In my implementation, Group has a Set<Subgroup> field. By default I initialize this with a HashSet<Subgroup>.
My Groups and Subgroups have an id field. I have implemented hashCode() and equals() in both classes to compare based on IDs. The ID is generated by the database (native generator in mapping). I base hashCode() and equals() off the ID because this makes sense for my application, and also because it represents Hibernate's concept of identity.
What I would like to do is create a Group, create some Subgroups in that Group, and then persist() only the Group, having it cascade save the new subgroups.
The problem I am facing is when I create a new Subgroup, its id field is 0 because it has not been persisted yet, and therefore the id field hasn't been set by Hibernate. Since all new Subgroups have an id of 0, they are equivalent (as per equals()) and thus only one can be stored in the Group's Set<Subgroup> set. This prevents me from adding Subgroups to Groups unless I persist() the Subgroups first to generate an ID and thus ensure uniqueness in the Group's set, but persisting the Subgroups first is counter to my objective of being able to only persist Groups and have it cascade to new Subgroups.
What is the right way to go about this?
Thanks!
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