Hibernate: Why Does `saveOrUpdate` Throw `NonUniqueObjectException` When Working with Many-to-Many Relationships?

Hibernate: Confusion with Multiple Objects of Same Identity

Issue:

When attempting to save or update an object using Hibernate's saveOrUpdate method, an exception is thrown:

org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session

Context:

The issue arises when an object, such as a RoleBean, is assigned to multiple entities, such as a UserBean and a GroupBean, within the same transaction. This can occur when the object has a many-to-many relationship.

For instance, in the given scenario, RoleBean#1 is initially assigned to GroupBean#1. When attempting to add it to UserBean#1 within the same session, the exception is encountered.

Resolution:

The cause of the exception is the existence of two different instances of the RoleBean object within the session. One instance is directly associated with the UserBean, while the other is indirectly associated through the GroupBean.

To resolve this issue, consider the following options:

1. Use merge instead of saveOrUpdate: The merge operation assumes that any changes made to a detached instance returned by Hibernate should be merged back into the database, regardless of identity. However, if the target object has been modified in between, merging may lead to data loss.
2. Ensure that the same instance of RoleBean is used throughout the transaction: This can be achieved by carefully assigning the correct instance to both the UserBean and the GroupBean. By maintaining a single instance, you avoid creating duplicate objects.
3. Consider using JPA's merge method: JPA provides a merge method that behaves similarly to Hibernate's merge. It assumes that any changes made to the detached instance should be merged back into the database.

By implementing one of these approaches, you can resolve the exception and ensure that objects with the same identity are properly handled within a single Hibernate session.

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