PostTag.java

@Immutable
@Entity
@Table(name = "post_tag")
@Getter
@EqualsAndHashCode
@NoArgsConstructor
public class PostTag {

    @NoArgsConstructor
    @AllArgsConstructor
    @EqualsAndHashCode
    @Embeddable
    public static class Id implements Serializable {

        private static final long serialVersionUID = 1L;        
        @Column(name = "post_id")
        protected UUID postId;
        @Column(name = "tag_id")
        protected UUID tagId;
    }

    @EmbeddedId
    protected PostTag.Id id = new PostTag.Id();

    @ManyToOne
    @JoinColumn(name = "post_id", insertable = false, updatable = false)
    private Post post;

    @ManyToOne
    @JoinColumn(name = "tag_id", insertable = false, updatable = false)
    private Tag tag;
    
    public PostTag(Post post, Tag tag) {
        this.post = post;
        this.tag = tag;
        this.id.postId = post.getId();
        this.id.tagId = tag.getId();
        post.getPostTags().add(this);
        tag.getPostTags().add(this);
    }