JPA - Generate a sequence id(non-unique) for each foreign key and make a composite key

Question

Scenario: I have two tables named Document and Element in my postgres db. The Relationship between them is one-to-many(one document has many elements). So the element table has a foreign key of document id. Now I need to have a sequence for the element ids starting from the initial value for each document id.

Eg :

e_id | d_id
------------
 1   |   x
 2   |   x
 3   |   x
 1   |   y
 2   |   y
 3   |   y

Once I have this, There is a next level relationship between element and another table called Labeled Element which has a composite key of Composite Key(document_id, element_id) and labeler_id.

Questions:

1. How to generate the sequence to start off from the initial value for each document id?

2. How do I show the JPA Mapping of the composite key in the label table. Specifically which field I map the document id too?

Here are the models for your convenience(In Scala):

Element

@Embeddable
class CompositeKey extends Serializable {

  @BeanProperty
  @Column(name = "id", nullable = false)
  var id : Long = _

  @BeanProperty
  @Column(name = "document_id", nullable = false)
  var documentId : UUID = _

  def this(id: Long, documentId: UUID) = {
    this()
    this.id = id
    this.documentId = documentId
  }
}

@Entity
@DynamicUpdate
@Table(name = "element")
class Element {

  @EmbeddedId
  @BeanProperty
  @GeneratedValue(strategy = GenerationType.SEQUENCE)
  var id: CompositeKey = _

  @MapsId("documentId")
  @BeanProperty
  @JoinColumn(name = "document_id", referencedColumnName = "id")
  @ManyToOne(fetch = FetchType.LAZY)
  var document: Document = _

LabeledElement

@Embeddable
class LabeledElementKey extends Serializable {
  @BeanProperty
  @Column(name = "document_id", nullable = false)
  var documentId : UUID = _

  @BeanProperty
  @Column(name = "labeler_id", nullable = false)
  var labelerId : Long = _

  @BeanProperty
  @Column(name = "element_id", nullable = false)
  var elementId : Long = _

  def this(documentId : UUID, labelerId : Long, elementId : Long) = {
    this()
    this.documentId = documentId
    this.labelerId = labelerId
    this.elementId = elementId
  }
}

@Entity
@Table(name = "labeled_element")
class LabeledElement {
  @EmbeddedId
  @BeanProperty
  @Column(unique = true)
  var id : LabeledElementKey = _

  @MapsId("labelerId")
  @JoinColumn(name = "labeler_id", referencedColumnName = "id")
  @ManyToOne(fetch = FetchType.LAZY)
  var labeler: Labeler = _

  @MapsId("elementId")
  @JoinColumns(Array(
    new JoinColumn (name = "element_id", referencedColumnName = "id"),
    new JoinColumn (name = "document_id", referencedColumnName = "document_id")
  ))
  @ManyToOne(fetch = FetchType.LAZY)
  var element: Element = _
// goes on

with the way it is right now, I get the following error :

org.hibernate.AnnotationException: No identifier specified for entity: model.Element

Your Efforts will be appreciated. Thanks!

Answer 1

您的Element类具有注释@Embeddable而不是@EmbeddedId 。