Merging an object with FetchType.EAGER relation leads to "FailedObject"

I have an entity VM with a relationship to another entity BP. The relationship is eagerly fetched. First I load a VM. After loading the VM is detached, serialized and changed at the client side. Now I want to update the changed entity so I use the EntityManager.merge() method from JPA. Now I run into the following error from OpenJPA:

"Encountered new object in persistent field "Vm.bp" during attach. However, this field does not allow cascade attach. Set the cascade attribute for this field to CascadeType.MERGE or CascadeType.ALL (JPA annotations) or "merge" or "all" (JPA orm.xml). You cannot attach a reference to a new object without cascading."

Why do I have to add a Cascade.MERGE to a relationship to another entity that will never change? And why does JPA think that BP is a new object ("...cannot attach reference to a new object...")? When using ManyToOn开发者_开发技巧e relationships do I always have to add Cascade.MERGE in order to update the entity or is this because of the EAGER fetch type?

Here's my entity:

@Entity
@Table(name = "VM")
public class Vm extends BaseEntity implements Serializable {
    public static final long serialVersionUID = -8495541781540291902L;
    @Id
    @SequenceGenerator(name = "SeqVm", sequenceName = "SEQ_VM")
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "SeqVm")
    @Column(name = "ID")
    private Long id;
    // lots of other fields and relations

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "BP_ID")
    private Bp bp;

    // ...
}

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I found the reason why this error message comes up: The @Version annotated database field of the related Bp entity was initialized with "0". Apparently OpenJPA (1.2.3) is not able to cope with entity versions of zero.

Setting the version to 1 solved my issue.