How to Fix Hibernate's 'Object References an Unsaved Transient Instance' Error?

Fixing Hibernate "Object References an Unsaved Transient Instance" Error

The "object references an unsaved transient instance" error in Hibernate occurs when trying to save an object with unsaved related objects without properly specifying cascading operations.

Cause:

This error arises when an entity has a collection property that contains new instances that have not yet been saved to the database. Hibernate attempts to automatically save these new instances when the parent object is persisted, but fails if the cascade attribute is not set appropriately.

Solution:

To resolve this issue, you need to specify the cascade attribute on your collection mapping. This tells Hibernate to save the related objects automatically when the parent object is saved.

If using XML mapping, add the following to your collection mapping:

<set name="relatedObjects" inverse="true" cascade="all">
    <key column="parent_id"/>
    <one-to-many class="RelatedObject" column="related_id"/>
</set>

If using annotations, add the following to your collection property:

@OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
private Set<RelatedObject> relatedObjects;

By specifying cascade=all, you instruct Hibernate to persist the related objects along with the parent object in a single transaction. This ensures that all necessary objects are saved to the database and the "unsaved transient instance" error is avoided.

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