我有实体项和单元，单元中的项是一对多相关的，我很难将数据输入到项中，其中项需要单元上的主键，而我不明白jcombobox如何获取主键或id。
项目类实体

@Entity()
@Table(name = "item")
public class Item extends Record{   

    @Column(unique = false, nullable = false, length = 100)
    public String name;

    @Column(unique = false, nullable = false, length = 50)
    public double stock; // untuk menambah terus data jumlah  item dari supplier

    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "unit_id")
    public Unit unit;

类单位实体

@Entity()
@Table(name = "unit")
public class Unit extends Record {

    @Column(unique = false, nullable = false, length = 100)
    public String name;
    @Column(unique = false, nullable = false, length = 100)
    public String description;

  @OneToMany(mappedBy = "unit", cascade = { CascadeType.PERSIST, CascadeType.MERGE, CascadeType.DETACH,
      CascadeType.REFRESH })
    private List<Item> item;

方法为组合框加载数据

private void getUnitItem() {

        List EntityUnitItem = new Unit(databaseSession).all();
        for (Iterator iterator = EntityUnitItem.iterator(); iterator.hasNext();) {

            Model.Entity.Unit getUnitItem = (Model.Entity.Unit) iterator.next();
            cmbSatuanItem.addItem(getUnitItem.name);

        }

    }

如何在按钮上保存一对多？