I am working on an existing domain model and using Hibernate Annotations. I have problem mapping an entity and need some help/inputs/ideas/suggestions.

Here is the scenario, I have 4 tables:
1) Oraganization (Org_id(primary key), Org_num, Org_name, etc...)
2) Office (Off_id (primary key), Org_id, Off_name, etc...)
3) Service (Svc_id (primary key), Svc_desc)
4) Office_Service (Off_id, Svc_id)

The issue is with mapping the SERVICES attribute in Organization entity. In my Organization entity I want list of service and my services are not directly linked to Organization. I have to write a custom SQL query to get the list, but I am not sure how to map. My Organization entity is some thing like this

@Entity
@Table(name = "Oraganizations")
public class EmploymentNetwork implements Serializable {

@Id
@Column(name = "Org_id")
private String orgId;

@Column(name = "Org_num")
private String orgNumber;

@OneToMany
//????
private List<Service> services;
}

I can't use JOINS, NAMEDNATIVEQUERY or....

Hope there is a way to map the scenario, looking forward for suggestions and ideas.

I tried the following mapping to address the above issue

@Entity
@Table(name = "Oraganizations")
@NamedNativeQuery(name = "Services", query = "SELECT.............................", resultClass = Service.class)
public class EmploymentNetwork implements Serializable {

@Id
@Column(name = "Org_id")
private String orgId;

@Column(name = "Org_num")
private String orgNumber;

@OneToMany
@Loader(namedQuery = "Services")
@JoinColumn(name = "Org_id")
private List<Service> services;
}

When I tried this mapping I am getting an exception

Exception in thread "main" java.lang.NullPointerException
at org.hibernate.collection.PersistentBag.size(PersistentBag.java:248)

Does anyone know what the issue?