JPA-多通级联

我有两个实体

ServiceDownload.java

@Entity
public class ServiceDownload implements Serializable {
    private static final long serialVersionUID = -5336424090042137820L;

@Id
@GeneratedValue
private Long id;
@Length(max = 255)
private String description;

private byte[] download;

private String fileName;

@ManyToOne
private Service service;

Service.java

@Entity
public class Service implements Serializable {
    private static final long serialVersionUID = 4520872456865907866L;


@EmbeddedId
private ServiceId id;

@Length(max = 255)
private String servicename;

@Column(columnDefinition = "text")
private String highlightsText;
@Column(columnDefinition = "text")
private String detailsText;
@Column(columnDefinition = "text")
private String productText;
@Column(columnDefinition = "text")
private String dataText;

@ManyToMany(mappedBy = "services")
private Set<Machine> machines;

@OneToMany(targetEntity = ServiceDownload.class)
private List<ServiceDownload> serviceDownloads;

@OneToOne
private ServicePicture servicePicture;

当我创建一个新的ServiceDownload对象并尝试持久化它时,我收到了一个重复的密钥异常。似乎jpa试图在服务表中插入一个新的服务对象。我怎样才能禁止这种行为?