GAE + JPA + JsonMappingException + You have just attempted to access field - google-app-engine

Question

I'm new with JPA and GAE. I'm having some problems trying to retrive some entities to the front end. I copy mi entity code:
#Entity
public class User implements Serializable{
private static final long serialVersionUID = -7949567692618870100L;
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Key key;
private String userName;
private String email;
#OneToMany(fetch=FetchType.LAZY, cascade = CascadeType.ALL)
private List<User> friends;
#OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
private Credential Credential;
//getters & setters
}
The Credential entity only contains userName and pass:
#Entity
public class Credential implements Serializable{
private static final long serialVersionUID = -3277842132830057420L;
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Key key;
private String userName;
private String pass;
//getters and setters
}
I generate UserEndpoint with GAE Eclipse Plugin and then I added a method to simulate a Login, this is the method:
#ApiMethod(name = "CredentialLogin")
public User CredentialLogin(#Named("pass") String pass,#Named("userName") String userName){
EntityManager mgr = getEntityManager();
Query query = mgr.createQuery("SELECT FROM User u WHERE u.email = :email");
query.setParameter("email", userName);
User user = null;
try {
user = (User) query.getSingleResult();
if (!user.getCredential().getPass().equals(pass))
//INVALID LOGIN
}
finally {
mgr.close();
}
return user;
}
Debugging the code I get the correct User from the datastore. Doing "user.getCredential()" the Credential fetchs to the User entity. As the "friends" field has LAZY fetching when I return the User entity I will appear NULL (This is what I want).
After the return I'm getting this error:
"com.google.appengine.repackaged.org.codehaus.jackson.map.JsonMappingException: You have just attempted to access field "friends" yet this field was not detached when you detached the object. Either dont access this field, or detach it when detaching the object."
I don't want to put EAGER fetching to my "friends" attribute because the list could be really big.
I don't want to remove my "friends" getter and setter method, because without them I couldn't access to the field.
Somebody knows what should I do.
Thank you very much

Answer 1

You can change the enhancement process to call a listener instead of throwing an exception when an undetached field is accessed. See http://www.datanucleus.org/products/accessplatform_3_1/enhancer.html and "detachListener" option