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In this part of the page on Baeldung, in class CourseRegistration he is not using @MapsId("id") and he is not even using "referencedColumnName" in the JoinColumn annotation. This was not the case with previous examples on this page. I feel that MapsId and JoinColumn with referencedColumnName should have been used. If not why? He has used the above in all other examples in the same page.

Abhijeet
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2 Answers2

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I feel that MapsId and JoinColumn with referencedColumnName should have been used. If not why? He has used the above in all other examples in the same page.

In the example that you provided he uses the mappedBy attribute on the other side of the relationship, where OneToMany exists. This mappedBy attribute provides necessary information to JPA to understand how those 2 entities are related. Keep in mind entity fields represent columns in database and JPA knows what those columns are.

The entity that uses the mappedBy indicates that the other entity on the other side is the owner of that relationship between those 2 entities.

@ManyToOne does not have a mappedBy attribute since by default when you have @OneToMany together with @ManyToOne it is the side that has the @ManyToOne that is the owner of the relationship. So no mappedBy exists on @ManyToOne to short circuit this default mechanism.

But there is a way of making the entity that has the @ManyToOne to be the owner of the relationship. You have to also put above @ManyToOne the @JoinColumn(name = "ref", referencedColumnName = "ref2")

To sum up the example that you point to, uses the default mechanism where the side with the @ManyToOne is the owner of the relationship, and that is indicated directly from the other side with the mappedBy attribute on @OneToMany.

Bonus

@JoinColumn(name = "student_id") without referencedColumnName is used not to indicate the owner side of the relationship but just so the JPA is informed that the extra column that would be used as a foreign key would not have the same name as the field of the entity has. So it is just used to override the foreign key column name that JPA will use as foreign key which will not be the same with the java field name.

As for @MapsId check this answer to understand better how it is used

marc_s
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Panagiotis Bougioukos
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Both are not needed here for different reasons:

  • @MapsId is mostly used to share a primary key between entities. CourseRegistration is its own entity with its own primary key, thus it is not needed here. Here is a quote from the official documentation:

Designates a ManyToOne or OneToOne relationship attribute that provides the mapping for an EmbeddedId primary key, an attribute within an EmbeddedId primary key, or a simple primary key of the parent entity. The value element specifies the attribute within a composite key to which the relationship attribute corresponds. If the entity's primary key is of the same Java type as the primary key of the entity referenced by the relationship, the value attribute is not specified.

(https://docs.jboss.org/hibernate/jpa/2.1/api/javax/persistence/MapsId.html)

  • referencedColumnName uses the name of the primary key of the referenced table as default and thus is not explicitly needed:

Default (only applies if single join column is being used): The same name as the primary key column of the referenced table.

(https://docs.jboss.org/hibernate/jpa/2.1/api/javax/persistence/JoinColumn.html#referencedColumnName())

puelo
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