Need solution for following scenario in Hibernate many to many mapping - spring

Question

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);
}