How to Query Parent Class Without Loading Subclasses in Hibernate?

How do I perform a query against the parent class in a Hibernate table-per-subclass inheritance setup without initializing any subclass entities?

// Example of querying parent class in Hibernate
Session session = sessionFactory.openSession();
Query query = session.createQuery("FROM ParentClass");
List<ParentClass> results = query.getResultList();
session.close();

In a Hibernate table-per-subclass inheritance strategy, querying a parent class without loading its subclasses can be challenging due to the way Hibernate fetches data. This guide outlines effective methods for accomplishing this, ensuring efficient handling of your object-oriented database design.

//@Entity
//@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
//@Table(name = "parent")
//public abstract class ParentClass {...}
//
//Example to fetch parent class only:
//Query query = session.createQuery("SELECT p FROM ParentClass p");
//List<ParentClass> parents = query.getResultList();

Causes

• Hibernate automatically fetches subclasses when querying the parent class by default.
• Using eager fetching strategies may lead to unnecessary subclass initialization.
• Misconfigured entity mappings may cause undesired loading behavior.

Solutions

• Use a `SELECT` query to retrieve only fields from the parent class, avoiding subclass fetching.
• Configure your entity to use `@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)` to control fetching behavior.
• Consider using projections to map results directly to a DTO, skipping subclass loading.

Mistake: Forgetting to close the Hibernate session after querying, which may lead to memory leaks.

Solution: Always ensure to close the Hibernate session within a try-with-resources block or in a finally clause.

Mistake: Using eager fetching by default, leading to unnecessary data loading from subclasses.

Solution: Utilize lazy fetching where possible by configuring the mappings appropriately.