"Welcome To Ashok IT"				
			                  "Spring Boot and MicroServices"
                                          Topic : Spring Data JPA - Association
                                          Date  : 02/11/2024
                                            (Session - 58)
_____________________________________________________________________________________________________________________________
Last Session
============
* We Just working with "Association" in ORM Frameworks.

* One-to-One Association using annotation i.e.,@OneToOne(Mapping) & @JoinColumn(Physical Relation b/w DB Tables)

* We can build the One-to-One Association using two directions

                                              Not Possible                     Possible
   1) Uni-Direction (Navigation) :  Student <----------------> Login, Login <--------------> Student

         Example
         =======                
                 Class Student{

                 }

                 Class Login{
                   
                     @OneToOne(cascadeType="", fetch="")
                     @JoinColumn(name="student_id")
                     private Student student;
                 }

                                               Possible                         Possible
   2) Bi-Direction (Navigation)  :  Student <----------------> Login, Login <--------------> Student 

	 Example
         =======                
                 Class Student{

                     @OneToOne(cascadeType="", fetch="")
                     private Login login;

                 }

                 Class Login{
                   
                     @OneToOne(cascadeType="", fetch="")
                     @JoinColumn(name="student_id")
                     private Student student;
                 }

One-To-Many Association
=======================
* In One-To-Many Association (or) Mapping means one row in a table is mapped to multiple rows in another table.

* let us consider the few examples for better understanding for Realtime examples

        University     ------------->  Students

        ShoppingCart   ------------->  Items

        User           ------------->  Emails,MobileNos

        Post           ------------->  Comments

        Car            ------------->  Models

        Student       ------------->  Courses

* Inorder to work with this association we need to use the following annotation in entity class as "@OneToMany" from Hibernate.

* We can build Association in two modes

1) One-To-Many Uni-Direction Association
========================================
* As we discussed earlier in uni-direction association always we can build the relationship either parent to child (or) child   to   parent entities only i.e., one way direction only.

 Example 
 =======
   
   @Entity								@Entity
   @Table(name="ashokit_posts)						@Table(name="ashokit_comments)
   public Class Post{							public Class Comment{

      private int postId;						     private int commentId

      private String postName;						     private String commentInfo;

      private Date createdDate;						     private Date createdDate;

      @OneToMany							}
      @JoinColumn
      private List<Comment> comments = new ArrayList(); 
  }

Database Tables
===============
   1) ashokit_posts  >>>>>>>>>>>>>>> post_id,post_name,created_date

   2) ashokit_comments  >>>>>>>>>>>> comment_id,comment_info,created_date,post_id

* In the above two entity classes Post is an Parent Class but where as Comment as Child class.

* In the uni-directional only one side is possible to retreive the records either parent to child (or) child to parent.

* In @OneToMany Association default fetch strategy is "LAZY" means whenever parent is loaded the child object should not be       loaded.

1) targetEntity:
================
* This attribute specifies the target entity class of the association. In most cases, you don't need to use this attribute      because the target entity can be inferred from the field type. However, you might use it when dealing with generic types or   interfaces.

  For example, targetEntity = Book.class would explicitly specify that the target entity of the one-to-many association is                  the Book class.

2) orphanRemoval
================
* This attribute is a boolean flag that defines whether to remove orphaned child entities automatically when they are removed   from the association in the parent entity.

* When orphanRemoval = true, if a book is removed from the books list of an author and the author is persisted or updated,      the orphaned book will be automatically deleted from the database.

* If orphanRemoval = false (the default), removing a book from the books list will only disassociate it from the author, and    it will not be deleted from the database automatically.

Example
=======

Post.java
=========
package com.ashokit.entity;

import java.util.ArrayList;
import java.util.Date;
import java.util.List;

import jakarta.persistence.CascadeType;
import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.FetchType;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.JoinColumn;
import jakarta.persistence.OneToMany;
import jakarta.persistence.Table;
import jakarta.persistence.Temporal;
import jakarta.persistence.TemporalType;

@Entity
@Table(name="ashokit_posts")
public class Post {
	
	@Id
	@Column(name = "post_id")
	@GeneratedValue(strategy = GenerationType.AUTO)
	private int postId;
	
	@Column(name="post_title", nullable = false)
	private String postTitle;
	
	@Column(name="created_date",nullable = false)
	@Temporal(TemporalType.DATE)
	private Date createdDate;
	
	@Column(name="created_by",nullable = false)
	private String createdBy;
	
	//onetoMany association
	@OneToMany(targetEntity = Comment.class,
			   cascade = CascadeType.ALL,
			   orphanRemoval = true,
			   fetch = FetchType.EAGER)
	@JoinColumn(name="post_id")
	private List<Comment> comments = new ArrayList<>();
	

	public int getPostId() {
		return postId;
	}

	public void setPostId(int postId) {
		this.postId = postId;
	}

	public String getPostTitle() {
		return postTitle;
	}

	public void setPostTitle(String postTitle) {
		this.postTitle = postTitle;
	}

	public Date getCreatedDate() {
		return createdDate;
	}

	public void setCreatedDate(Date createdDate) {
		this.createdDate = createdDate;
	}

	public String getCreatedBy() {
		return createdBy;
	}

	public void setCreatedBy(String createdBy) {
		this.createdBy = createdBy;
	}
	
	public void setComments(List<Comment> comments) {
		this.comments = comments;
	}
	
	public List<Comment> getComments() {
		return comments;
	}

	@Override
	public String toString() {
		return "Post [postId=" + postId + ", postTitle=" + postTitle + ", createdDate=" + createdDate + ", 			        createdBy="+ createdBy + "]";
	}
}

Comment.java
============
package com.ashokit.entity;

import java.util.Date;

import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.Table;
import jakarta.persistence.Temporal;
import jakarta.persistence.TemporalType;

@Entity
@Table(name="ashokit_comments")
public class Comment {
	
	@Id
	@Column(name = "comment_id")
	@GeneratedValue(strategy = GenerationType.AUTO)
	private int commentId;
	
	@Column(name="comment_info", nullable = false)
	private String commentInfo;
	
	@Column(name="created_date",nullable = false)
	@Temporal(TemporalType.DATE)
	private Date createdDate;
	
	@Column(name="created_by",nullable = false)
	private String createdBy;

	public int getCommentId() {
		return commentId;
	}

	public void setCommentId(int commentId) {
		this.commentId = commentId;
	}

	public String getCommentInfo() {
		return commentInfo;
	}

	public void setCommentInfo(String commentInfo) {
		this.commentInfo = commentInfo;
	}

	public Date getCreatedDate() {
		return createdDate;
	}

	public void setCreatedDate(Date createdDate) {
		this.createdDate = createdDate;
	}

	public String getCreatedBy() {
		return createdBy;
	}

	public void setCreatedBy(String createdBy) {
		this.createdBy = createdBy;
	}

	@Override
	public String toString() {
		return "Comment [commentId=" + commentId + ", commentInfo=" + commentInfo + ", createdDate=" + createdDate
				+ ", createdBy=" + createdBy + "]";
	}
}

CommentDao.java
===============
package com.ashokit.dao;

import org.springframework.data.jpa.repository.JpaRepository;

import com.ashokit.entity.Comment;

public interface CommentDao extends JpaRepository<Comment, Integer>{

}


PostDao.java
============
package com.ashokit.dao;

import org.springframework.data.jpa.repository.JpaRepository;

import com.ashokit.entity.Post;

public interface PostDao extends JpaRepository<Post, Integer>{

}

application.yml
===============
 spring:
  datasource:
    driver-class-name: com.mysql.cj.jdbc.Driver
    password: root
    url: jdbc:mysql://localhost:3306/31_springbootms_db
    username: root
  jpa:
    hibernate:
      ddl-auto: update
    show-sql: true
    properties:
      hibernate:
        enable_lazy_load_no_trans: true

Application.java
================
package com.ashokit;

import java.util.Date;
import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

import com.ashokit.dao.CommentDao;
import com.ashokit.dao.PostDao;
import com.ashokit.entity.Comment;
import com.ashokit.entity.Post;

@SpringBootApplication
public class Application implements CommandLineRunner{
	
	@Autowired
	private PostDao postDao;
	
	@Autowired
	private CommentDao commentDao;

	public static void main(String[] args) {
		SpringApplication.run(Application.class, args);
	}
	
	@Override
	public void run(String... args) throws Exception {		
		
		//First Post Information
		Post post = new Post();
		post.setPostTitle("Spring Boot Post");
		post.setCreatedDate(new Date());
		post.setCreatedBy("Mahesh");
		
		//Binding the First comment for First Post
		Comment firstComment = new Comment();
		firstComment.setCommentInfo("Informative Session");
		firstComment.setCreatedDate(new Date());
		firstComment.setCreatedBy("Suresh");
		
		//Binding the Second comment for First Post
		Comment secondComment = new Comment();
		secondComment.setCommentInfo("Good Session");
		secondComment.setCreatedDate(new Date());
		secondComment.setCreatedBy("Ramesh");
		
		//parent to child saving records
		post.setComments(List.of(firstComment,secondComment));
		
		//saving the parent object
		Post postInfo = postDao.save(post);
		System.out.println("Post Information::::" + postInfo);
		
		//Second Post Information
		Post post1 = new Post();
		post1.setPostTitle("Devops With AWS Post");
		post1.setCreatedDate(new Date());
		post1.setCreatedBy("Ashok");
		
		//Binding the First comment for Second Post
		Comment firstPostComment = new Comment();
		firstPostComment.setCommentInfo("Informative Session");
		firstPostComment.setCreatedDate(new Date());
		firstPostComment.setCreatedBy("Suresh");
		
		//Binding the First comment for Second Post
		Comment secondPostComment = new Comment();
		secondPostComment.setCommentInfo("Good Session");
		secondPostComment.setCreatedDate(new Date());
		secondPostComment.setCreatedBy("Ashok");
		
		//Binding the First comment for Second Post
		Comment thirdPostComment = new Comment();
		thirdPostComment.setCommentInfo("Good Informative Session");
		thirdPostComment.setCreatedDate(new Date());
		thirdPostComment.setCreatedBy("Ashok");
		
		//setting parent to child object
		post1.setComments(List.of(firstPostComment,secondPostComment,thirdPostComment));
		
		Post savedFirstPostInfo = postDao.save(post1);
		System.out.println("Saved First Post Info ::::::" + savedFirstPostInfo);
		
		//loading the parent object
		Post post11 = postDao.findById(1).get();
		System.out.println("Loading Post Information ::::" + post11);		
		System.out.println("Loading Post  & Comment Information ::::" + post11.getComments());
		
		//deleting parent object associated child objects should be deleted
		postDao.deleteById(1);
	}
}