Ikram Ajay

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Hibernate Tips is a series of posts in which I describe a quick and easy solution for common Hibernate questions. If you have a question for a future Hibernate Tip, please post a comment below.

Himanshu Srivastava asked today’s question on my YouTube channel:

“What’s the difference between@JoinColumn and @PrimaryKeyJoinColumn? Also, should we use @PrimaryKeyJoinColumn or @JoinColumn?”

The @JoinColumn and the @PrimaryKeyJoinColumn might seem very similar, but they are used in 2 different contexts. You can use the @JoinColumn annotation to map the foreign key column of a managed association. The @PrimaryKeyJoinColumn specifies the mapping of the foreign key column of a secondary table or the foreign key column in an inheritance mapping that uses the JOINED strategy.

So, the annotation you need to use depends on the context in which you want to customize the mapping of the foreign key column. Let’s take a look at an example mapping for both annotations.

When you add the @JoinColumn annotation to your association mapping, you can define the name of the foreign key column that represents your association in the table model.

I use it in the following code snippet to tell Hibernate it shall use the column book_key as the foreign key column of the many-to-one association.

When you activate the logging of SQL statements and fetch a Review entity from the database, you can see that Hibernate uses the book_key column to map the association.

When you use the inheritance mapping strategy JOINED, all columns mapped by the superclass get mapped to one database table, and the specific attributes of a subclass get mapped to a different table. Hibernate has to join these 2 tables, whenever you use the subclass. The primary key columns of each table are then used as the join columns.

You can see the superclass of such an inheritance hierarchy in the following code snippet.

By default, Hibernate expects that the primary key columns of both tables have the same name. If that’s not the case, you can annotate your subclass with a @PrimaryKeyJoinColumn.

As you can see in the log message, Hibernate maps the primary key of the book table to the book_id column.

If you want to learn more about foreign key mappings, you should read the following articles:

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