difference of owned one-to-many relationship between GAE/J low level datastore API and DataNucleus JDO

Question

When creating an owned one-to-many relationship, in Java, I noticed that there is a difference in the resulting record between using the low level Datastore API and DataNucleus JDO. Not sure if this is intentional or any way to fix it.

For example,

If there are multiple addresses for an employee in the following link:

https://developers.google.com/appengine/docs/java/datastore/entities#Ancestor_Paths

Using the low level datastore api as following, the employee record doesn't show an address column(i.e. property):

Entity employee = new Entity("Employee");
datastore.put(employee);

Entity address_home = new Entity("Address", employee.getKey());
datastore.put(address_home);

Entity address_mailing = new Entity("Address", employee.getKey());
datastore.put(address_mailing);

Using JDO, the employee record shows an address column(i.e. property):

@PersistenceCapable
public class Employee {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent(mappedBy = "employee")
    private List<Address> addresses;

    List<Address> getAddresses() {
        return addresses;
    }
    void setAddresses(List<Address> addresses) {
        this.addresses = addresses;
    }

    // ...
}

@PersistenceCapable
public class Address {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent
    private Employee employee;

    @Persistent
    private String Street;

    ...
 }

The extra property is harmless. However why is this necessory for JDO?

I'm using GAE/J 1.7.2 with DataNucleus v2 on dev server.

Since you don't even quote the Address class, and its presumed relation to Employee, how can anyone answer that? — DataNucleus
Thanks. I thought Address class is not interesting. I will add it to the original question. — De Cai
Actually Address wasn't interesting but you had Employee with a List of "ContactInfo" previously, which you've also updated hence it makes sense now — DataNucleus

DataNucleus DataNucleus · Accepted Answer · 2012-09-30T06:21:00

Latest storage version of the GAE JDO plugin will store all relations in an object as a property, hence the Employee class will have a property for the addresses it stores. This is a much more logical way of storing things than how GAE JDO used to store things (it originally tried to simulate foreign keys, using the ownership of the other side). Storing the List in the owner has advantages for loading the elements into the collection, as well as allowing an element to be present in a List more than once (whereas with older storage versions that would be impossible).

All version of GAE JDO upto and including 2.1.1 store the index position in each Address that is in the List, whereas really they need not be stored now since the "addresses" property in Employee provides that - this is left over from the early versions which needed it stored in that way. Version 2.1.2 onwards don't add that list index property to the element.

difference of owned one-to-many relationship between GAE/J low level datastore API and DataNucleus JDO

1 Answers