How To Populate a Child-Side Parent Association via Proxy

Description: A Hibernate proxy can be useful when a child entity can be persisted with a reference to its parent (@ManyToOne or @OneToOne association). In such cases, fetching the parent entity from the database (execute the SELECT statement) is a performance penalty and a pointless action, because Hibernate can set the underlying foreign key value for an uninitialized proxy.

Key points:

• rely on EntityManager#getReference()
• in Spring, use JpaRepository#getOne() -> used in this example
• in Hibernate, use load()
• assume two entities, Author and Book, involved in a unidirectional @ManyToOne association (Author is the parent-side)
• we fetch the author via a proxy (this will not trigger a SELECT), we create a new book, we set the proxy as the author for this book and we save the book (this will trigger an INSERT in the book table)

Output example:

• the console output will reveal that only an INSERT is triggered, and no SELECT

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