Relationships and Cascade types. @OneToOne, @OneToMany, @ManyToMany relationships

@OneToOne 

Method1.

package com.example.lesson_8_parvin.entity;

import lombok.Data;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;
import javax.persistence.Table;
import java.util.ArrayList;
import java.util.List;

@Data
@Entity
@Table(name = "post")
public class Post {
    @Id
    @GeneratedValue
    private Long id;
    private String title;

    @OneToMany(mappedBy = "post", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Comment> comments = new ArrayList<>();

    @OneToOne(mappedBy = "post", cascade = CascadeType.ALL,
            fetch = FetchType.LAZY, optional = false)
    private Detail detail;
    
}

package com.example.lesson_8_parvin.entity;

import lombok.Data;

import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToOne;
import javax.persistence.Table;

@Data
@Entity
@Table(name = "detail")
public class Detail {

    @Id
    @GeneratedValue
    private Long id;
    private String detail;

    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "post_id")
    private Post post;
}

package com.example.lesson_8_parvin;

import com.example.lesson_8_parvin.entity.Detail;
import com.example.lesson_8_parvin.entity.Post;
import com.example.lesson_8_parvin.repository.CommentRepository;
import com.example.lesson_8_parvin.repository.PostRepository;
import lombok.RequiredArgsConstructor;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
@RequiredArgsConstructor
public class Lesson8ParvinApplication implements CommandLineRunner {

    private final PostRepository postRepository;
    private final CommentRepository commentRepository;

    public static void main(String[] args) {
        SpringApplication.run(Lesson8ParvinApplication.class, args);
    }

    @Override
//    @Transactional
    public void run(String... args) throws Exception {
        Detail detail = new Detail();
        detail.setDetail("detail");
        Post post = new Post();
        post.setTitle("title");
        post.setDetail(detail);
        detail.setPost(post);
        postRepository.save(post);
    }
}

@OneToMany

Method1.

Post(parent) - Comment(child)

package com.example.lesson_8_parvin.entity;

import lombok.Data;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import java.util.ArrayList;
import java.util.List;

@Data
@Entity
@Table(name = "post")
public class Post {
    @Id
    @GeneratedValue
    private Long id;
    private String title;

    @OneToMany(mappedBy = "post", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Comment> comments = new ArrayList<>();
}

package com.example.lesson_8_parvin.entity;

import lombok.Data;
import lombok.ToString;

import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Data
@Entity
@Table(name = "comment")
public class Comment {

    @Id
    @GeneratedValue
    private Long id;
    private String review;

    @ToString.Exclude
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "post_id")
    private Post post;
}

package com.example.lesson_8_parvin;

import com.example.lesson_8_parvin.entity.Comment;
import com.example.lesson_8_parvin.entity.Post;
import com.example.lesson_8_parvin.repository.CommentRepository;
import com.example.lesson_8_parvin.repository.PostRepository;
import lombok.RequiredArgsConstructor;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

import java.util.Arrays;
import java.util.List;

@SpringBootApplication
@RequiredArgsConstructor
public class Lesson8ParvinApplication implements CommandLineRunner {

    private final PostRepository postRepository;
    private final CommentRepository commentRepository;

    public static void main(String[] args) {
        SpringApplication.run(Lesson8ParvinApplication.class, args);
    }

    @Override
//    @Transactional
    public void run(String... args) throws Exception {
        Comment one = new Comment();
        one.setReview("one");
        Comment two = new Comment();
        two.setReview("two");
        Comment three = new Comment();
        three.setReview("three");
        List<Comment> list = Arrays.asList(one, two, three);

        Post post = new Post();
        post.setTitle("title");
        post.setComments(list);
        one.setPost(post);
        two.setPost(post);
        three.setPost(post);
        postRepository.save(post);
    }
}

@ManyToMany

Cascade types