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How does a @OneToOne relationship work in Spring Data JPA?

great read. simple explanations. funny though how the first one is the only one worth exploring if you ask me!

Before jumping into @OneToOne examples, be sure to understand the basics of <a id=0 href='https://www.stackchief.com/blog/Spring%20Data%20JPA'>Spring Data JPA</a>.

1) @OneToOne Example Using Shared Primary Key (Best Approach)

    @GeneratedValue(strategy = GenerationType.AUTO)

    @OneToOne(mappedBy = "author", cascade = CascadeType.ALL)

This creates a one-to-one relationship between an author and book table. A book has one author and an author has one book.

Using the @OneToOne annotation, Hibernate knows to create a one-to-one relationship between the two entities.

The mappedBy="author" option tells Hibernate that the owning side of the relationship is the book table. This means the book table will hold the foreign key reference to the author table.

The cascade = CascadeType.ALL tells Hibernate to propagate changes to all associated entities. If you delete an instance of Author it will delete the associated Book entity automatically.

The @PrimaryKeyJoinColumn tells Hibernate that the primary key for the author table will be used as the foreign key in the owning book table.

The @MapsId tells Hibernate that the primary key on the author table will be used as the primary key for the book table. Since the primary key is being reused, we don't need a to specify @GenderatedValue on the book table.

This generates the following in the database...

When retrieving entities from the database, associations will be eagerly loaded by default:

Author author = authorRepo.findById(24) //returns both the author and its associated book in single DB call

By utilizing the primary key of the author as both the primary key and foreign key of the book table, you achieve the same result with fewer keys. This saves operating costs as primary/foreign keys are typically indexed and loaded into memory for efficient lookups. You're achieving the same result with fewer keys via a unidirectional relationship.

With this approach, the author table is the owning side of the relationship. This means the author table will have the foreign key book_id. For these reasons, the mappedBy = is moved to the Book entity definition.

Since the primary key is no longer being reused in both tables, the Book entity must now generate its own id values. For these reasons, the @GeneratedValue annotation exists on both entity classes.

Note: Unlike the last example, we now save the author entity to create the associations in the database. Whichever side owns the relationship must be saved to create entries in both tables.. Otherwise, you will run into issues with transient objects not being saved and exceptions being thrown.

Notice how the author table now has 2 columns.  This is because a foreign key is being used to create a bidirectional relationship between books and authors.

This example clearly demonstrates the advantage of the first approach as an extra book_id is needed to achieve the relationship.

                    { @JoinColumn(name = "author_id", referencedColumnName = "id") },

                    { @JoinColumn(name = "book_id", referencedColumnName = "id") })

One to One Example using Spring Data JPA and Hibernate including shared primary key, unidirectional, and bidirectional examples.

One To One Example | Spring Data JPA

CascadeType is an option that specifies how related entities should be treated...

let's say you have a one-to-many relationship between courses and students...

in your @Entity class you may have something like:

This is saying that every operation you perform on this entity, every associated entity will get the same treatement,

So if you delete a parent entity all of it's children (associations) will also get deleted.

Sometimes this makes sense. Sometimes it doesn't.  BE CAREFUL in using this as it can have unexpected consequences...

What is the purpose of CascadeType in Spring Data JPA / Hibernate?

Can we call garbage collector forcefully in Java

EatFreshRupesh

Is Kafka a programming language?

What is event driven architecture?

Is Java losing popularity?

How much do software engineers make?

How much do software engineers make?

What is Qualifier in Spring Boot?

What does SpringApplication.run() do?

What is the Log4J2 security vulnerability?

How does a @OneToOne relationship work in Spring Data JPA?

One To One Example | Spring Data JPA

What is the purpose of CascadeType in Spring Data JPA / Hibernate?

Can we call garbage collector forcefully in Java?

EatFreshRupesh

Is Kafka a programming language?

What is event driven architecture?

Is Java losing popularity?

How much do software engineers make?

How much do software engineers make?

What is Qualifier in Spring Boot?

What does SpringApplication.run() do?

What is the Log4J2 security vulnerability?

How does a @OneToOne relationship work in Spring Data JPA?

One To One Example | Spring Data JPA

What is the purpose of CascadeType in Spring Data JPA / Hibernate?

Can we call garbage collector forcefully in Java?

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