Post.kt

@Entity
@Table(name = "post")
data class Post(
    @get:Id
    @get:GeneratedValue
    @get:Column(name = "id")
    val id: Long,

    @get:Column(name = "title")
    val title: String,

    @get:ManyToMany
    @get:JoinTable(
            name = "post_tag",
            joinColumns = [JoinColumn(name = "post_id")],
            inverseJoinColumns = [JoinColumn(name = "tag_id")]
    )
    val tags: Set<Tag> = HashSet()
)

Tag.kt

@Entity
@Table(name = "tag")
data class Tag(
    @get:Id
    @get:GeneratedValue
    @get:Column(name = "id")
    val id: Long,

    @get:Column(name = "name")
    val name: String,

    @get:ManyToMany(mappedBy = "tags")
    val posts: Set<Post> = HashSet()
)