Data JPA-Collections Mapping

Posted On: 2023-03-26 16:11:51 Spring Boot

*) For every collection one child table is created
*) This child table contains at max 3 columns
   Key Column - FK Column
   Index Column - Position  (only for List/Map)
   Element Column -- Data

*) @ElementCollection Should be provided over collecton variable.
*) To provide child table details created for collection variable
   like table name, FK(Join Column)..etc

   @CollectionTable(
                name="",
		joinColumns = @JoinColumn(name="")
   )

*) Element Column name : @Column(name="")

*) For List Collection we should add : 
   @OrderColumn(name="")
   List stores data in indexs given by java (0,1,2,3...)

*) For Map Collection we should add :
   @MapKeyColumn(name="")
   Map - Key behaves like index
   Map - value behaves like element column.

---------code-------------------------------------------------------
Name: SpringBoot2DataJpaCollectionsEx
Dep : Data JPA, MySQL, Lombok

1. Model class
package in.nareshit.raghu.model;

import java.util.List;
import java.util.Map;
import java.util.Set;

import javax.persistence.CollectionTable;
import javax.persistence.Column;
import javax.persistence.ElementCollection;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.MapKeyColumn;
import javax.persistence.OrderColumn;
import javax.persistence.Table;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@NoArgsConstructor
@AllArgsConstructor
@Entity
@Table(name="emp_tab")
public class Employee {
	
	@Id
	@Column(name="eid")
	private Integer empId;
	@Column(name="ename")
	private String empName;
	@Column(name="esal")
	private Double empSal;
	
	
	//collection variables
	@ElementCollection
	@CollectionTable(
			name="emp_prjs_tab",
			joinColumns = @JoinColumn(name="eid")
			)
	@Column(name="proj")
	private Set empPrjs;
	
	
	@ElementCollection
	@CollectionTable(
			name="emp_modules_tab",
			joinColumns = @JoinColumn(name="eid")
			)
	@OrderColumn(name="epos")
	@Column(name="module")
	private List empModules;
	
	
	@ElementCollection
	@CollectionTable(
			name="emp_clients_tab",
			joinColumns = @JoinColumn(name="eid")
			)
	@MapKeyColumn(name="cindex")
	@Column(name="cname")
	private Map<String,String> empClients;
	
	
}


2. Repository
package in.nareshit.raghu.repo;
import org.springframework.data.jpa.repository.JpaRepository;
import in.nareshit.raghu.model.Employee;

public interface EmployeeRepository
	extends JpaRepository<Employee, Integer>
{

}

3. Runner class
package in.nareshit.raghu.runner;

import java.util.List;
import java.util.Map;
import java.util.Set;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;

import in.nareshit.raghu.model.Employee;
import in.nareshit.raghu.repo.EmployeeRepository;

@Component
public class TestRunner implements CommandLineRunner {
	
	@Autowired
	private EmployeeRepository repo;
	
	public void run(String... args) throws Exception {
		repo.save(
				new Employee(10, "A", 3.3, 
						Set.of("P1","P2","P3"), 
						List.of("M1","M2"), 
						Map.of("C1","NIT","C2","ORCL")
						)
				);
		System.out.println("DONE");
	}

}

*) proeprties file
#datasource
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver
spring.datasource.url=jdbc:mysql://localhost:3306/boot8pm
spring.datasource.username=root
spring.datasource.password=root
#JPA 
spring.jpa.show-sql=true
spring.jpa.hibernate.ddl-auto=create
spring.jpa.database-platform=org.hibernate.dialect.MySQL8Dialect
----------------------------------------------------------
@ElementCollection  (must)

--child table --
 table name  @CollectionTable(name="")

 KeyColumn - @JoinColum(name="")

 Index Column
   List  -  @OrderColumn(name="")
   Map   -  @MapKeyColumn(name="")

 Element Column  - @Column(name="")
-----------------------------------------------------------
Task:
Student
 sid
 sname
 sfee
 marks:List
 subnames:Set
 grades:Map<String,String>

*) Child table created for collection never contains extra columns
  this is not association mapping.

*) If we do not provide Collectiontable and JoinColumn
then 
  tablename is = className_variableName
  Join Column Name is = className_pkColumnName

ie Providing 
   @CollectionTable(name="__",
      joinColumns = @JoinColumn(name="__"))
are optional.

@JoinColumn(name="") -- FK Column created for linking other table PK
=================================================

class mapped with table
variable --- column
collection variable --- childtable
object  -- row