Caused by: Exception [EclipseLink-7155] (Eclipse Persistence Services - 2.2.0.v20110202-r8913): org.eclipse.persistence.exceptions.ValidationException
Exception Description: The type [class inventory.entity.Item] for the attribute [item] on the entity class [class sale.entity.ItemSold] is not a valid type for a serialized mapping. The attribute type must implement the Serializable interface.

@Entity
public class ItemSold extends ModelObject implements Serializable {
	
	/**
	 * 
	 */
	private static final long serialVersionUID = -2793958653008272733L;
	@Id
	@GeneratedValue(strategy=GenerationType.AUTO)
	private Integer id;
	private Integer quantity;
	@Column(precision=6,scale=2)
	private BigDecimal price;
	@Column(precision=6,scale=2)
	private BigDecimal discount;
	private Item item;
	@ManyToOne
	private Sale sale;
	
	public ItemSold() {
	}
        ...
}

@Entity
public class Item extends ModelObject implements Serializable {
		
	/**
	 * 
	 */
	private static final long serialVersionUID = -7789306711121155171L;
	@Id
	@GeneratedValue(generator="ItemTab")
	@TableGenerator(name="ItemTab",table="SEQUENCE",
			pkColumnName="SEQ_NAME",valueColumnName="SEQ_COUNT",
			pkColumnValue="ITEM_GEN",allocationSize=1)
	private Integer id;
	private String code;
	private String key;
	private String description;
	private Preferences brand;
	private Preferences category;
	@Column(precision=6,scale=2)
	private BigDecimal buyPrice=BigDecimal.valueOf(0);
	@Column(precision=6,scale=2)
	private BigDecimal previousBuyPrice=BigDecimal.valueOf(0);
	@Column(precision=6,scale=2)
	private BigDecimal salePrice=BigDecimal.valueOf(0);
	@Column(precision=6,scale=2)
	private BigDecimal minimumSalePrice=BigDecimal.valueOf(0);
	private Integer stock=0;
	private Integer threshold=0;
	private Integer sold=0;
	private Integer bought=0;
	@ManyToMany(cascade=CascadeType.PERSIST,fetch=FetchType.EAGER,targetEntity=inventory.entity.Year.class)
	private List<Year> years;

       ...
}