Fetchgroups usage

[MALOLETNIY, Igor (Igor)** CTR **]

Hi, I have a question about fetch groups in openJPA.

I have two entities: User and Device.

User:
@Entity
public class CmsUser extends CmsObject {

    @Basic
    private String name;

    @Basic
    private String login;

    //search for join table field as list<long>
    @OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.PERSIST)
    @JoinColumn(name = "user_id", referencedColumnName = "id")
    private List<Device> devices;

    public CmsUser(String name, String login) {
        this.name = name;
        this.login = login;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getLogin() {
        return login;
    }

    public void setLogin(String login) {
        this.login = login;
    }

    public List<Device> getDevices() {
        return devices;
    }

    public void setDevices(List<Device> devices) {
        this.devices = devices;
    }

    public CmsUser() {
    }

    @Override
    public String toString() {
        return "id = " + getId()
                + ", login = " + getLogin()
                + ", devices = " + getDevices();
    }
}

Device :

@Entity
@FetchGroup(name = "flat", attributes = {@FetchAttribute(name = "deviceId")})
public class Device extends CmsObject {

    @Basic
    private String directoryNumber;

    @Basic
    private String deviceId;

    @Enumerated(EnumType.STRING)
    private CmsType type;

    public Device(String directoryNumber, String deviceId, CmsType type) {
        this.directoryNumber = directoryNumber;
        this.deviceId = deviceId;
        this.type = type;
    }

    public String getDirectoryNumber() {
        return directoryNumber;
    }

    public void setDirectoryNumber(String directoryNumber) {
        this.directoryNumber = directoryNumber;
    }

    public String getDeviceId() {
        return deviceId;
    }

    public void setDeviceId(String deviceId) {
        this.deviceId = deviceId;
    }

    public CmsType getType() {
        return type;
    }

    public void setType(CmsType type) {
        this.type = type;
    }

    public Device() {
    }

    @Override
    public String toString() {
        return "id = " + getId()
                + ", number = " + getDirectoryNumber()
                + ", type = " + getType();
    }
}

SuperClass Object:
@MappedSuperclass
public abstract class CmsObject {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }
}

How can I implement feature that loads User with devices which contains only id 
field?

I have test class that loads users. When user's devices fetch=lazy 
users.device=null. And when fetch=eager It loads all device fields.