In this article, we will explore how to build a many-to-many relationship in Spring Boot Rest and JPA. We’ll also look at the different options available for achieving this goal and which approach is better suited for your data model.
In this blog post, we will be discussing how to create a many-to-many relationship in Spring Boot Rest + JPA.
We’ll learn about the Many-to-Many connection in JPA and how to apply it in a Spring Boot application in this post. Let’s get this party started!
Annotation @ManytoMany
When numerous records in one table are linked to several records in another table, this is known as a many-to-many connection. A many-valued connection with many-to-many multiplicity is defined by the @ManytoMany annotation. According to the official documents:
There are two sides to every many-to-many relationship: the owning side and the non-owning, or inverse, side. On the owning side, the join table is defined. If the connection is bidirectional, the non-owning side must provide the relationship field or attribute of the owning side using the mappedBy element of the ManyToMany Annotation.
Is it better to be unidirectional or bidirectional?
To represent Many to Many Relationships in JPA, we utilize the @ManyToMany annotation. It might be either one-way or two-way communication.
- Only one entity points to another in a unidirectional relationship.
- Both elements in a bidirectional relationship point to each other.
Example of an Employees Portal Illustration
Let’s look at an Employees Portal for a University as an example. At the same time, an employee may be both a teacher and a chairman. Similarly, the roles of Teacher and Chairman might be distributed among multiple personnel from various departments. A Many-to-Many connection works like this.
To comprehend the Many-to-Many connections in the situation under examination, examine the two entities Users and Role. As a result, below is the most basic Entity-Relationship Diagram:
Structure of the Database
The PostgreSQL database is used as the RDMS in our example. On a related point, have you tried establishing a table in Postgres with the name “User”? It wouldn’t work since the keyword in Postgres is User!
In the ER Diagram, we have a users table with id as the Primary Key, as well as a table role with id as the Primary Key. A join table named users roles is also responsible for linking the two sides using the Foreign Keys user id and role id.
Set Up the Project Structure
In any of your preferred IDEs, create a Maven, Spring Boot project. For further information, see this article.
pom.xml
4.0.0 spring-boot-starter-parent org.springframework.boot manytomanyrelationship 2.5.6 com.library org.springframework.boot spring-boot-starter-test test org.springframework.boot spring-boot-starter-data-jpa org.springframework.boot spring-boot-starter-web org.postgresql.org.postgresql.org.postgresql.org.postgresql.org.postgresql.org.postgresql.org.postgresql.org.postgresql.org.post org.springframework.boot spring-boot-maven-plugin postgresql runtime
application.properties
pring.datasource.url=jdbc:postgresql:/localhost:5432/postgres spring.datasource.username=test spring.datasource.password=test spring.datasource.driverClassName=org.postgresql.Driver spring.jpa.generate-ddl=true spring.jpa.hibernate.ddl-auto = create spring.jpa.show-sql= true spring.jpa.hibernate.ddl-auto = create spring.jpa.show-s
Make a list of the Domain Model Classes.
Using JPA Annotations, create model classes for Users and Roles. Spring and JPA will take care of the remainder during execution.
Users
import javax.persistence.*; import java.util.Collection; package com.manytomanyrelationship.domain; Users is a public class in @Entity. Integer id; String username; String password; @Id @GeneratedValue(strategy = GenerationType.AUTO) Integer id; String username; String password; @JoinTable @ManyToMany (name = “users roles”), joinColumns = @JoinColumn(name = “user id”, referencedColumnName = “id”), inverseJoinColumns = @JoinColumn(name = “role id”, referencedColumnName = “id”), inverseJoinColumns = @JoinColumn(name = “role id”, reference roles in private collections; Users from the general public () ‘ ‘ this.username = username; this.password = password; this.roles = roles; public Users(String username, String password, Collection roles) this.username = username; this.password = password; this.roles = roles; return username with public String getUsername(); setUsername(String username) public void this.username = username; return password; public String getPassword(); setPassword(String password) public void this.password = password; getRoles() returns roles in a public Collection; setRoles(Collection roles) public void this.roles = roles;
Role
package com.manytomanyrelationship.domain; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.GeneratedType; import javax.persistence.Id; @Entity; @Entity; @Entity; @Entity; @Entity; @Entity; @Entity; @Entity; @Entity; @Entity; @Entity; @Entity; @Id @GeneratedValue(strategy = GenerationType.AUTO) public class Role public Role; integer id; string name (Integer id, String name) this.name = name; this.id = id; super(); this.name = name; public Role(String name) TODO: auto-generated constructor stub public Role() return name; public String getName(); this.name = name; public void setName(String name)
The role is a basic Model class that contains no User information. We have, however, utilized the @ManyToMany annotation in a Users object, resulting in an Unidirectional Many to Many Mapping. Furthermore, navigation is limited to one side.
- The @Entity annotation indicates that the classes are JPA entities.
- In both classes, the attribute id is annotated with @Id and @GeneratedValue annotations. To begin with, the @Id annotation indicates that this is the main key. The main key generation strategy is defined by the later annotation, which has the strategy type AUTOINCREMENT.
- The @ManyToMany annotation is applied to the List of Role attribute in the Users class, indicating a many-to-many relationship between the two entities. Furthermore, using CascadeType. ALL ensures cascade, which implies that the persistence of Users tuples is followed by the persistence of Role tuples.
- The @JoinTable annotation is used in addition to the @ManyToMany annotation. The @JoinTable annotation is used to create a users role table, which connects the Users and Roles entities. In our example, the parameter joinColumn will keep the main key of this Users object (the Owning side), while inverseJoinColumns will contain the primary key of the other entity (the Inverse of the Relationship).
Create a Rest Controller.
UsersController
Import com.manytomanyrelationship.service.UsersService; import org.springframework.beans.factory.annotation; package com.manytomanyrelationship.rest. Import org.springframework.http.ResponseEntity; import org.springframework.web.bind.annotation. Autowired; import org.springframework.http.ResponseEntity; import org.springframework.http.ResponseEntity; import org.springframework Import org.springframework.web.bind.annotation for RequestMapping. @RestController public class; RestController; RestController; RestController; RestController; RestController; Rest @Autowired public UsersService usersService; @RequestMapping(“/getUsers”) public ResponseEntity UsersController
Watch This Video-
A many-to-many relationship is a relationship between two different entities that are not necessarily related. In Spring Boot Rest + JPA, you can fetch data in this type of relationship by using the @JoinTable annotation. Reference: how to fetch data in many-to-many relationship in jpa.
Frequently Asked Questions
How does JPA handle many-to-many relationships?
A: In JPA, you will create a composite many-to-many relationship which involves two joins. One join is against the navigation table and the other one is from that same navigation table to another table.
How would you implement many-to-many relationship in spring boot?
A: A number of different ways. Some common implementations are through the use of a join table, or by using a relationship class that knows about other classes and automatically implements all relationships for you. In your case, if you want to be able to have many-to-many relationships between two entity types in Spring boot, you would need an @JoinTable annotation on one type with another type as its value expression field.
How do you map a many-to-many relationship?
A: You would do this by using a database of the many-to-many relationships.
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