How To Map An Entity To a Query (@Subselect) in a Spring Boot Application

Note: Consider using @Subselect only if using DTO, DTO and extra queries, or map a database view to an entity is not a solution.

Description: This application is an example of mapping an entity to a query via Hibernate, @Subselect. Mainly, we have two entities in a bidirectional one-to-many association. An Author has wrote several Book. The idea is to write a read-only query to fetch from Author only some fields (e.g., DTO), but to have the posibility to call getBooks() and fetch the Book in a lazy manner as well. As you know, a classic DTO cannot be used, since such DTO is not managed and we cannot navigate the associations (don’t support any managed associations to other entities). Via Hibernate @Subselect we can map a read-only and immutable entity to a query. This time, we can lazy navigate the associations.

Key points:

• define a new entity that contains only the needed fields from the Author (including association to Book)
• for these fields, define only getters
• mark the entity as @Immutable since no write operations are allowed
• flush pending state transitions for the used entities by @Synchronize
• use @Subselect to write the needed query, map an entity to an SQL query

---

If you need a deep dive into the performance recipes exposed in this repository then I am sure that you will love my book "Spring Boot Persistence Best Practices"	If you need a hand of tips and illustrations of 100+ Java persistence performance issues then "Java Persistence Performance Illustrated Guide" is for you.

---