How to Resolve Hibernate's MultipleBagFetchException During SessionFactory Creation?

MultipleBagFetchException during Hibernate SessionFactory Creation

When encountering the "MultipleBagFetchException: cannot simultaneously fetch multiple bags" exception during SessionFactory creation, it's crucial to inspect the entities involved.

In the provided example, Parent contains an OneToMany relationship to Child children. Additionally, Parent's children list is eagerly fetched. However, when including another "parent" entity AnotherParent into Parent, an error arises due to the presence of two eagerly fetched collections.

To resolve this issue, instead of using FetchType.EAGER, consider annotating the collection fields with @LazyCollection(LazyCollectionOption.FALSE). This approach avoids the eager fetching of the collections and eliminates the exception.

It's worth noting that in most cases, a Set is more appropriate than List for representing the relationship between Parent and Child. Unless a specific need for a List exists, using a Set can help mitigate the aforementioned issue.

While this solution addresses the exception, it's important to proceed with caution. Using a Set alone does not completely eliminate the potential for a Cartesian Product, as described by Vlad Mihalcea in their response labeled as "The worst solution."

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