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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

I agree that redux was a solution built to solve react issues but conceptually it's just a state management tool which I think can apply to some angular applications. I haven't worked extensively with different angular redux flavors but I have worked a lot with ngrx. I completely agree that it adds an enormous amount of boilerplate code but in certain circumstances that boilerplate becomes a powerful framework of state management. In most applications state can and should be managed in services and alternative persistent storage etc. However in complex applications that rely on pulling from large data sources in real-time and share that data across the application I'd argue that a redux style store is incredibly beneficial. At some point with large applications, storing your state in services just becomes you rewriting the existing functionality of a redux style library. I get that dispatching actions for every little thing can become tedious but I think if you build your store thoughtfully it will benefit you in the end.

Jake Leveroni

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A look at the rise of Redux and misconceptions surrounding its need in the Angular world.

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it is absolutely imperative that you use SimpleChanges with ngOnChanges.

i think the way Angular handles change events is much simpler than ReactJS.  While these ngonchange examples may seem a bit overboard for JavaScript, it's still easier to understand these life cycle hooks than functional components in React (in my opinion).

Does anyone know what is more performant when it comes to things like React hooks (useEffect, useState) etc and things like ngOnChanges???

Steve Jennings

People often get confused when they see ngOnChanges fire before ngOnInit. At a high level you would assume ngOnInit is the first thing called when a component is rendered.

This post does a good job of emphasizing the fact that ngOnChanges is linked to children components and their bound @Input properties.  When you initialize a child component from a parent, you are setting the component's @Input properties as part of the component's initialization....

A child component's @Input properties are updated even before the component is initialized...

hoopmiester

ngOnChanges example for Angular including latest version example with explanation

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Using Java 8, 9, or 11 is the best best in 2021...

Well. My company standard it's openJDK 8 since 2019... Cheers!

juanrios.com@localhost

thank you, I finally realize that Java 8 is the most popular version.

Evgeny Ivanov

A discussion surrounding the different versions of Java including the difference between SE and EE as well as key features of Java 7, 8, and 9.

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Code247

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

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