Spring Boot One To Many Example (2024)

In this tutorial, we'll demonstrate how to create Spring Boot + JPA + One to Many Unidirectional + Bidirectional mapping with MySQL database.

Spring Boot One To Many Unidirectional Example

The unidirectional One-To-Many association is easier because it only specifies the relationship on the parent side. We only use the @OneToMany annotation at parent side in unidirectional mapping.

1. If we do not use @JoinColumn to define foreign key column, Hibernate maps the relationship using an association table and create one more table to define association.
2. Now three tables will be generated instead of two, which will take additional storage and Memory to index the Foreign Keys.

  @OneToMany
  private Set<Employees> employees = new HashSet<>();

Refer detail implementation with source code for Spring Boot One To Many Unidirectional Example.

To prevent this extra table, we must define the foreign key column (org_id) with a @JoinColumn annotation.

 @OneToMany
 @JoinColumn(name = "org_id", referencedColumnName = "id")
 private Set<Employees> employees = new HashSet<>();

Spring Boot One To Many Bidirectional Example

A bidirectional association enables us to retrieve information about the dependent object from both sides of the relation.

Refer detail implementation with source code for Spring Boot One To Many Bidirectional Example.

Q: What is association mapping in JPA?
Ans:

An important main components of JPA and Hibernate is association mapping. It represent the relationship between domain model attributes with database tables attributes. This mapping enables the user to manage the relationship with ease by using JPQL or Criteria Queries.

Q: How many types of association mapping are there in Hibernate?
Ans:

The four ways that JPA and Hibernate provide the mapping of associations between objects are listed below. Both unidirectional and bidirectional association mappings are possible.

1. Many-to-One
2. One-to-One
3. One-to-Many
4. Many-to-Many

Q: What is one to many relationship in Spring JPA?
Ans:

One to many mapping describes the ability to map a single row in one table to multiple rows in another table. For example, consider an organization where we have employees. A company may have a lot of employees, hence this is a one to many mapping.

Q: What is a unidirectional and bidirectional relationship in Spring JPA?
Ans:

A bidirectional association enables us to retrieve information about the dependent object from both sides while a unidirectional relationship is only valid in one direction.

Test Spring Boot JPA One To Many Example

1. Start the Spring Boot Application by running spring-boot:run or by running main class.
2. Open Postman, use POST method with end point http://localhost:8080/org/assign and provide organization and employees details.
3. Once the above records are generated, the following tables and data will be generated:
4. Employees table with its data.

Refer Spring Boot JPA CRUD Example to implement all the CRUD rest endpoints with this example.