JPA Many-To-One Mapping

Java Persistence API (JPA) is a specification that simplifies database interaction in Java applications. A Many-To-One relationship represents an association where multiple instances of one entity are linked to a single instance of another entity.

For example, many students can belong to one university. Here, Student is the owning side of the relationship, while University is the referenced entity.

Many-to-One Mapping

In a Many-To-One relationship:

• Multiple child entities reference a single parent entity.
• The foreign key column is maintained in the child entity’s table.
• The mapping is achieved using the @ManyToOne annotation.

Example: Each Student belongs to one University, but one University can have many Students.

Project Implementation

Goal

The goal of this project is to demonstrate how to establish a Many-To-One relationship in JPA using annotations.
We will map multiple Student entities to a single University entity and persist this relationship in a relational database using Hibernate as the JPA provider.

Process Overview

The implementation involves the following steps:

1. Set up a JPA project with Hibernate and MySQL dependencies.
2. Configure the persistence unit using persistence.xml.
3. Define entity classes University and Student.
4. Establish a Many-To-One relationship using @ManyToOne and @JoinColumn.
5. Persist and retrieve entities through an EntityManager.

Step 1: Create a JPA Project

Create a new Maven project named jpa-many-to-one-mapping-demo in your IDE (e.g., IntelliJ IDEA or Eclipse).

Project Structure:

👁 onemanyfile
Step 2: Add Dependencies (pom.xml)

Step 3: Configure Persistence Unit (persistence.xml)

Location: src/main/resources/META-INF/persistence.xml

Step 4: Create the University Entity

Step 5: Create the Student Entity

• @ManyToOne: defines that many students belong to one university.
• @JoinColumn: specifies the foreign key column (university_id) in the student table.

Step 6: Create the Main Class

• It creates an EntityManagerFactory and EntityManager to interact with the database.
• A University entity is created and persisted, followed by multiple Student entities linked to that university.
• The transaction is committed to save all records permanently in the database.
• Finally, a student is retrieved and its associated university is printed, demonstrating the Many-To-One relationship.

Step 7: Output

Console Output:

👁 Output in Console

Database Tables will be created.