Using @OneToMany annotation in JPA

Suppose, you are working on a project where the database has two tables, such as student and clazz, structured like below:

As you can see, the clazz table has columns id and name with the primary key id. Student table has columns id, name and clazz_id with primary key id. In this student table, we have also defined a foreign key from it to the clazz table, meaning that a class can have more than one student, and many students can be classed. In JPA, to express the relationship of many students with the same class, we will use the @ManyToOne annotation, and to express a class relationship with many students, we will use the @OneToMany annotation.

We have learned about annotation @ManyToOne in the previous tutorial. This tutorial I will be with you to learn about annotation @OneToMany.

First, I will create a Maven project to illustrate for the above example:

Using @OneToMany annotation in JPA

I will add Hibernate dependency to use JPA as follows:

In this example, I am working with MySQL so I will also add the MySQL Driver dependency as follows:

Next, I will create the Clazz and Student entities to display their information.

Using @OneToMany annotation in JPA

The content of the Clazz entity is as follows:

and entity Student:

Now, we will use the @OneToMany annotation to show the relationship of the clazz table to the student table in JPA.

Because, this is a one-to-many relationship seen from the Clazz object so I’ll put the @OneToMany annotation in the Clazz object as follows:

As you can see, in this annotation, I have declared an attribute mappedBy with a value is clazz. The “clazz” value here is the variable name defined with the @ManyToOne annotation in the Student entity.

Here, we have completed using the @OneToMany annotation to define the Clazz entity. I will try to see how it works.

I will add a configuration file, persistence.xml:

Using @OneToMany annotation in JPA

with the following content:

Suppose, now in the database, I have the following data:

Using @OneToMany annotation in JPA

With this code:

The results will look like below:




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