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Spring Data JPA unable to persists data to DB

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posted 4 months ago

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Hi guys.
I am learning Spring boot and I am having problem with persisting data to DB.

I am trying to create a simple blog, with post and category. Here are the models.

Post

package com.psquared.blog.model;

import com.fasterxml.jackson.annotation.JsonBackReference;
import com.fasterxml.jackson.annotation.JsonManagedReference;
import jakarta.persistence.*;

@Entity
public class Post {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

@Column(length = 255)
    private String title;

@ManyToOne(cascade = CascadeType.ALL)
    private Category category;

@Column(columnDefinition = "TEXT")
    private String content;

public int getId() {
        return id;
    }

public void setId(int id) {
        this.id = id;
    }

public String getTitle() {
        return title;
    }

public void setTitle(String title) {
        this.title = title;
    }

public String getContent() {
        return content;
    }

public void setContent(String content) {
        this.content = content;
    }

public Category getCategory() {
        return category;
    }

public void setCategory(Category category) {
        this.category = category;
    }

@Override
    public String toString() {
        return "Post{" +
                "id=" + id +
                ", title='" + title + '\'' +
                ", content='" + content + '\'' +
                '}';
    }
}

Category

Then I created a command line runner as follows:

package com.psquared.blog;

import com.psquared.blog.model.Category;
import com.psquared.blog.model.Post;
import com.psquared.blog.service.CategoryService;
import com.psquared.blog.service.PostService;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.transaction.annotation.Transactional;

import java.util.List;
import java.util.Optional;

@SpringBootApplication
public class BlogApplication {

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

@Bean
	@Transactional
	public  CommandLineRunner commandLineRunner(PostService postService, CategoryService categoryService){
		return (args -> {

Category java = new Category();
			java.setName("java");

Post p1 = new Post();
			p1.setTitle("post 1");
			p1.setContent("content");
			p1.setCategory(java);

Post p2 = new Post();
			p2.setTitle("post 2");
			p2.setContent("content 2");
			p2.setCategory(java);

postService.save(p1);
			postService.save(p2);

});
	}

}

I am try to save two posts and the related category to db, but for some reason only one post and related category is persisting, the second post is not persisting at all.

Further, Here is the error I am getting:

I've been at it for 3 days now. Please help.

Tim Holloway

Saloon Keeper

Posts: 27868

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posted 4 months ago

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I can't tell you anything without seeing what PostService does.

I can tell you that JPA Entities can exist in one of 2 states: attached or detached.

An attached Entity has invisible properties that connect it to the EntityManager that attached it. A detached Entity does not. You can only post changes when an object is attached. If it is detached, you must do a merge() to re-attach it.

Note that the merge method does not (cannot!) simply write in the invisible attachment properties. Instead it returns an entirely new instance of that Entity. So one you merge, forget about using the pre-merge instance for anything. Discard it and work with what the merge returns.

Or, in other words:

Now e1.equals(e2) is true, but "e1 == e2" is false and further work should be done on e2, ignoring e1.

You error message indicates that you've attempted to commit (in postService) an instance that was detached.

The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.

You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.

With a little knowledge, a cast iron skillet is non-stick and lasts a lifetime.