JPA In A Nutshell

jpa-in-a-nutshell.md

JPA In A Nutshell

Identifiers

Assigned

Generated

Entity State Transitions

Entity States

• New (Transient)
• Persistent (Managed)
• Detached
• Removed

https://vladmihalcea.com/a-beginners-guide-to-jpa-hibernate-entity-state-transitions/

Persist And Merge

Persist

• must only be used for new entities
• attaches an entity to the currently running persistence context

Merge

• is required only for detached entities
• reattaches the entity to a new Persistence Context

https://vladmihalcea.com/jpa-persist-and-merge/

Associations

Unidirectional @OneToMany

class Post(
    @Id
    @GeneratedValue(strategy = GenerationType.UUID)
    var id: UUID? = null,
    val title: String,
    @OneToMany(
        cascade = [CascadeType.ALL], 
        orphanRemoval = true
    )
    @JoinColumn("post_id")
    val comments: MutableList<Comment> = mutableListOf()
    val content: String
)

class Comment(
    @Id
    @GeneratedValue(strategy = GenerationType.UUID)
    var id: UUID? = null,
    val content: String
)

insert into post (content,title,id) values (?,?,?)
insert into comment (content,id) values (?,?)
insert into comment (content,id) values (?,?)
insert into comment (content,id) values (?,?)
update comment set post_id=? where id=?
update comment set post_id=? where id=?
update comment set post_id=? where id=?

Bidirectional @OneToMany

@Entity
class Post(
    @Id
    @GeneratedValue(strategy = GenerationType.UUID)
    var id: UUID? = null,
    val title: String,
    val content: String,
    @OneToMany(
        mappedBy = "post",
        cascade = [CascadeType.ALL],
        orphanRemoval = true
    )
    val comments: MutableList<Comment> = mutableListOf()
) {
    fun addComment(comment: Comment) {
        comments.add(comment)
        comment.post = this
    }

    fun removeComment(comment: Comment) {
        comments.remove(comment)
        comment.post = null
    }
}

@Entity
class Comment(
    @Id
    @GeneratedValue(strategy = GenerationType.UUID)
    val id: UUID? = null,
    @ManyToOne(fetch = FetchType.LAZY)
    var post: Post? = null,
    val content: String
) {
    override fun equals(other: Any?): Boolean {
        if (this === other) return true
        if (javaClass != other?.javaClass) return false

        other as Comment

        return id != null && id == other.id
    }

    override fun hashCode(): Int = javaClass.hashCode()
}

insert into post (content,title,id) values (?,?,?)
insert into comment (content,post_id,id) values (?,?,?)
insert into comment (content,post_id,id) values (?,?,?)
insert into comment (content,post_id,id) values (?,?,?)

Unidirectional @ManyToOne

class Post(
    @Id
    @GeneratedValue(strategy = GenerationType.UUID)
    var id: UUID? = null,
    val title: String,
    val content: String
)

class Comment(
    @Id
    @GeneratedValue(strategy = GenerationType.UUID)
    var id: UUID? = null,
    @ManyToOne(fetch = FetchType.LAZY)
    val post: Post,
    val content: String
)

Unidirectional @OneToOne

@Entity
class Detail(
    @Id
    var id: UUID? = null,
    val createdAt: LocalDateTime,
    val createdBy: String,
    @OneToOne(fetch = FetchType.LAZY)
    @MapsId
    val post: Post
)

https://vladmihalcea.com/the-best-way-to-map-a-onetoone-relationship-with-jpa-and-hibernate/#:~:text=The%20best%20way%20to%20map%20a%20%40OneToOne%20relationship%20is%20to,using%20the%20Post%20entity%20identifier.&text=This%20way%2C%20the%20id%20property,Primary%20Key%20and%20Foreign%20Key. https://vladmihalcea.com/the-best-way-to-map-a-onetomany-association-with-jpa-and-hibernate/