Converter only in one direction?

[quilian]

Hi!

short version: Is it somehow possible to have a Converter accepting an MyEntity Instance as input and get only instance.getId (String) out of it to store in the model?

long version:
I'm trying to write my own converter. I have a the two classes User and Employee:

USER is managed by Hibernate and therefore an Entity :

@Entity(optimisticLock = OptimisticLockType.DIRTY, dynamicUpdate = true)
@Cache(usage = CacheConcurrencyStrategy.TRANSACTIONAL)
@javax.persistence.Entity
@Table(name = "MB_USER", uniqueConstraints = @UniqueConstraint(columnNames = "KENNUNG"))
public class Benutzer
          implements
               java.io.Serializable,
               HibernateManagedEntity {

     private static final long serialVersionUID = 1L;

     protected BigDecimal id;
     protected String kennung;
     protected String kennwort;
     protected String employeeid;
...

EMPLOYEE is just a dummy class which gets filled with proper values from 'external' (actually it's the same db, but it could be any other system):

public class Employee extends ExternalEntity implements java.io.Serializable {
     protected String id;
     protected String label;

     // Getters and Setters
     ...

The property User.employeeid is semantically a foreign key but without the db constraint for technical reasons.
When editing a User, I want to select the Employees like this:

<h:selectOneMenu id="xy" value="userManager.instance.employeeid">
    <s:selectItems id="select" value="employeeManager.resultList" var="emp" label="#{emp.label}"/>
    <mb:convertEntity managerName="employeeManager"/>
</h:selectOneMenu>

where the mb:convertEntity is my own implementation similiar to s:convertEntity:

Name("com.mobilanten.mowita.werkbank.converter.EntityConverter")
@Scope(CONVERSATION)
@Converter
@BypassInterceptors
public class EntityConverter
          implements
               javax.faces.convert.Converter,
               Serializable {
     private static final long serialVersionUID = 1L;

     private String managerName;
     private EntityStore store;

     @Create
     public void create() {
          store = EntityStore.instance();
     }

     private void init() {
          if (getManager() != null) {
               store.setManager(getManager());
          } else
               throw new IllegalStateException(
                         "manager not defined. you must define the attribute 'managerName' for tag mb:entityConverter!");
     }

     @SuppressWarnings("unchecked")
     @Transactional
     public String getAsString(FacesContext facesContext, UIComponent cmp,
               Object value) throws ConverterException {
          init();
          if (value == null) {
               return null;
          }
          if (value instanceof String) {
               return value.toString();
          } else          
          if (value instanceof Entity) {
               return ((Entity) value).getId().toString();
          } else
               throw new ClassCastException(
                         "Value to convert is not a derivative of com.mobilanten.mowita.werkbank.Entity but "
                                   + value.getClass().getName());
     }

     @Transactional
     public Object getAsObject(FacesContext facesContext, UIComponent cmp,
               String value) throws ConverterException {
          init();
          if (value == null) {
               return null;
          }
          return value; // instead of store.get(value), which would give me the real entity; i want the id only
     }

     @SuppressWarnings("unchecked")
     public Manager<Entity> getManager() {
          return (Manager<Entity>) Component.getInstance(getManagerName());
     }

     public String getManagerName() {
          return managerName;
     }

     public void setManagerName(String aManagerName) {
          managerName = aManagerName;
     }

This gives me the expected html output with employee.id as value and label as label:

..
 <option value="287">Kilian,Tobias</option>
...

Now there is no Employee member in User, only the String.
So I changed getAsObject(..) to only return the id. This gives me a facelets error with the content of
javax.faces.component.UISelectOne.INVALID

As a workaround i thought about extending User with something like

public setEmployeeByInstance( Employee employee) {
    setEmployee( employee.getId());
}

but I hope this can be avoided....

Thanks in advance for any help!