Wsrtjtyk

Consider the tables where posts and tags exhibit a many-to-many relationship between each other.

The many-to-many relationship is implemented using a third table called post_tags which contains the details of posts and their associated tags.

Post Model



@Entity

@Table(name = "posts")

public class Post {

    @Id

    @GeneratedValue(strategy = GenerationType.IDENTITY)

    private Long id;



    @NotNull

    @Size(max = 100)

    @Column(unique = true)

    private String title;



    @NotNull

    @Size(max = 250)

    private String description;



    @NotNull

    @Lob

    private String content;



    @NotNull

    @Temporal(TemporalType.TIMESTAMP)

    @Column(name = "posted_at")

    private Date postedAt = new Date();



    @NotNull

    @Temporal(TemporalType.TIMESTAMP)

    @Column(name = "last_updated_at")

    private Date lastUpdatedAt = new Date();





    @ManyToMany(fetch = FetchType.LAZY,

            cascade = {

                CascadeType.PERSIST,

                CascadeType.MERGE

            })

    @JoinTable(name = "post_tags",

            joinColumns = { @JoinColumn(name = "post_id") },

            inverseJoinColumns = { @JoinColumn(name = "tag_id") })

    private Set<Tag> tags = new HashSet<>();





    public Post() {



    }



    public Post(String title, String description, String content) {

        this.title = title;

        this.description = description;

        this.content = content;

    }



    // Getters and Setters (Omitted for brevity)

}

TAG Model



@Entity

@Table(name = "tags")

public class Tag {

    @Id

    @GeneratedValue(strategy = GenerationType.IDENTITY)

    private Long id;



    @NotNull

    @Size(max = 100)

    @NaturalId

    private String name;



    @ManyToMany(fetch = FetchType.LAZY,

            cascade = {

                CascadeType.PERSIST,

                CascadeType.MERGE

            },

            mappedBy = "tags")

    private Set<Post> posts = new HashSet<>();



    public Tag() {



    }



    public Tag(String name) {

        this.name = name;

    }



    // Getters and Setters (Omitted for brevity)

}

Problem is

I tried to use an existing tags. and insert happened only on posts & posts_tags table.

Initially i'm Get tag(s) with tagName(s). Once you have the Tag object, you can set it in the Post object and save it.

Like this

 Post post = new Post("Hibernate Many to Many Example with Spring Boot",

                "Learn how to map a many to many relationship using hibernate",

                "Entire Post content with Sample code");



        // Create two tags

        Tag tag1 = tagService.getTag("Spring Boot");





        // Add tag references in the post

        post.getTags().add(tag1);



        postRepository.save(post);

If I do like that, entry is not available in post_tags table.

Tag Repository and Tag Service:

@Repository

public interface TagRepository extends JpaRepository<Tag, Long> {



    @Query("select p from Tag p where p.name = :name")

    Tag findByName(@Param("name") String name);

}



@Override

public Tag findByName(String name) {

  return repository.findByName(name);

}