Can I persist child objects in @PrePersist handler of a parent class? (Objectify 3.1b1)

Question

I am new to Objectify and trying to implement One-to-Many relationship. I have entities Organization and entity Person. Organization has @Transient property List< Person > contactPeople. Class Person has @Parent property Key< Organization > organizationKey which I can set via setter.

I'd like to persist contactPeople in @PrePersist handler of Organization. In order to do that I need to set parent key in Person.

Wiki here says: "You can't update @Id or @Parent fields in a @PrePersist callback; by this time, the low-level Entity has already been constructed with a complete Key so it can be passed in as an optional parameter."

I'm not sure this is still accurate information ? Because key of com.google.appengine.api.datastore.Entity object that I get in PrePersist handler has key that literally says "no-id-yet".

How would you implement this ?

Thank you!

Update at Nov 17, 2011:

In new Objectify4 we'll be able to do semi-automatic relationships like this:

class Beastie {
   @Parent
   @Load
   ParentThing parent;

   @Id Long id;

   @Load({"bigGroup", "smallGroup"})
   SomeThing some;

   @Load("bigGroup")
   List<OtherThing> others;

   @Load
   Ref<OtherThing> refToOtherThing;

   Ref<OtherThing> anotherRef;  // this one is never fetched automatically
}

Here is evolving design document of new version.

This is big news. Twig author, John Patterson, joined Objectify project today.

Out of curiosity I did this: @PrePersist void onPersist(Objectify ofy) { if (id == null) id = ofy.getFactory().allocateId(getClass()); }. And it turned out this allocated ID is overridden by number which is higher that I've allocated which leads me to the conclusion that Wiki is outdated and Key is being constructed after PrePersist event. Sad. — expert
Jeff Schnitzer, author of Objectify responded to my question. "I generally think it's a bad idea to put persistence operations inside entity pojos. This is one reason why. That information is accurate. You can't do what you are proposing. I can't think of a workaround offhand. Jeff" — expert

Igor Artamonov Igor Artamonov · Accepted Answer · 2011-10-20T06:11:13

Hm, seems that you need to make an Dao in front of your data models. So, you will able to do something like:

Organization organization = ...
List<Person> people = ...
ob.put(organization)
for (Person person: people) {
    person.organizationKey = organization.getKey();
    ob.put(person);
    organization.contactPeopleKeys.add(person.getKey());
}
ob.put(organization)

GAE+Objectify requires a lot of thing to handle by your own code, so it's a common thing

Can I persist child objects in @PrePersist handler of a parent class? (Objectify 3.1b1)

1 Answers