Spring Boot Microservices application example with step by step guide for beginners

Intoroduction

In this tutorial we will see how to use ElementCollection to implement one to many mapping in spring boot application. In our previous example we have used one-to-many mapping, but here we are using ElementCollection fron JPA which is a very simple than regular one-to-many mapping soluion. We are going to learn how to use these two annotations @ElementCollection and @CollectionTable.

Database Schema

Consider a simple example where Product has multiple manufacturers and suppliers. Here we are going to create three database tables named as product, manufacturer and supplier

Database schema for product

Dependencies to add in pom file

we need to add web, jpa and mysql driver dependencies into pom file

<dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-web</artifactId>
  </dependency>

  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-data-jpa</artifactId>
  </dependency>

  <dependency>
   <groupId>mysql</groupId>
   <artifactId>mysql-connector-java</artifactId>
   <scope>runtime</scope>
  </dependency>

Database related properties

Then we need to set configurations in application.properties file for hibernate and mysql connection

# Database connection details
spring.datasource.url=jdbc:mysql://localhost:3306/jai43?useSSL=false&serverTimezone=UTC&useLegacyDatetimeCode=false
spring.datasource.username=jai43user
spring.datasource.password=123456


# hibernate as JPA imlementation

spring.jpa.properties.hibernate.dialect = org.hibernate.dialect.MySQL5InnoDBDialect

# this to let the spring boot to perfrom database schema update whene there is a changes in entity class
spring.jpa.hibernate.ddl-auto=update

Creating Embeddable model Classes

This is the major step here we can see the differences than regular one-to-mapping and ElementCollection. We are going to create Manufacturer and Supplier model classes as embeddable class type.

Creating embeddable model class

By using @Embeddable a class level annotation can be used to create embeddable model class

@Embeddable
public class Manufacturer {
 @NotNull
 String manufacturerName;

 @NotNull
 String mlocation;

 public Manufacturer(@NotNull String manufacturerName, @NotNull String mlocation) {
  super();
  this.manufacturerName = manufacturerName;
  this.mlocation = mlocation;
 }

 public String getManufacturerName() {
  return manufacturerName;
 }

 public void setManufacturerName(String manufacturerName) {
  this.manufacturerName = manufacturerName;
 }

 public String getMlocation() {
  return mlocation;
 }

 public void setMlocation(String mlocation) {
  this.mlocation = mlocation;
 }
}

For supplier database table we are not going to create a separate model class, because it has only one field other than foreign key, so in a person entity class directly we can map with String type.

Creating Entity class for parent table ( product table )

Here product table is a parent table for manufacturer and supplier table. We are going to use @ElementCollection and @CollectionTable  annotations to establish one to many mapping.

@Entity
@Table(name = "product")
public class Products {
 
 @Column(name = "productid")
 @Id
 @GeneratedValue(strategy = GenerationType.IDENTITY)
 Long pid;
 
 @Column(name = "productname")
 @NotNull
 @Size(max = 100)
 String pname;
 
 @Column(name = "productdesc")
 @NotNull
 @Size(max = 100)
 String pdesc;

 
 // here we are going to use ElementCollection to enable Supplier table
 @ElementCollection
 @CollectionTable(name = "supliers", joinColumns = @JoinColumn(name="productid"))
 @Column(name = "supliername")
 Set<String> suppName= new HashSet<String>();
 
 // ElementCollection for Manufacturer table with embeddable model class
 @ElementCollection
 @CollectionTable(name = "manfacturer",joinColumns = @JoinColumn(name = "productid"))
 @AttributeOverrides({  
  @AttributeOverride(name = "manufacturerName", column = @Column(name="mname"))
 })
 Set<Manufacturer> man = new HashSet<Manufacturer>();
 
 public Products( @NotNull @Size(max = 100) String pname, @NotNull @Size(max = 100) String pdesc) {
  super();
  
  this.pname = pname;
  this.pdesc = pdesc;
 }

 public Products( @NotNull @Size(max = 100) String pname, @NotNull @Size(max = 100) String pdesc,
   Set<String> suppName, Set<Manufacturer> man) {
  super();
  
  this.pname = pname;
  this.pdesc = pdesc;
  this.suppName = suppName;
  this.man = man;
 }
}

Repository for products

@Repository
public interface ProductRepository extends JpaRepository<Products, Long> {

}

Inserting operation

String pname="Phone";
String pdesc="Mobile phone for calling";
    
Set<String> suppls= new HashSet<String>();
suppls.add("Samsung");
suppls.add("Apple");

Set<Manufacturer> man= new HashSet<Manufacturer>();
man.add(new Manufacturer("Samsung", "India"));

// Without inserting values into supplier and manufacturer table
Products p= new Products(pname, pdesc);//, suppls, man);
pr.save(p);

// With insertion of supplier and manufacturer
Products p= new Products(pname, pdesc, suppls, man);
pr.save(p);

output

output of one to many mapping with ElementCollection

Further Reading

One to many mapping example with Spring Boot and JPA, MYSQL as a Database