Question

I am in the process of moving an existing Google AppEngine application from the master-slave datastore (MSD) to the new high-replication datastore (HRD).
The application is written in Java, using Objectify 3.1 for persistence.

In my old (MSD) application, I have an entity like:

public class Session {
  @Id public Long id;
  public Key<Member> member;
  /* other properties and methods */
}

In the new (HRD) application, I have changed this into:

public class Session {
  @Id public Long id;
  // HRD: @Parent is needed to ensure strongly consistent queries.
  @Parent public Key<Member> member;
  /* other properties and methods */
}

I need the Session objects to be strongly consistent with their parent Member object.

When I migrate (a working copy of) my application using Google’s HRD migration tool, all Members and Sessions are there. However, all member properties of Session objects become null. Apparently, these properties are not migrated.

I was prepared to re-parent my Session objects, but if the member property is null, that is impossible. Can anyone explain what I am doing wrong, and if this problem can be solved?

Answer 1

@Id and @Parent are not “real” properties in the underlying entity. They are part of the key which defines the entity; Objectify maps them to properties on your POJO.

The transformation you are trying to make is one of the more complicated problems in GAE. Remember that an entity with a different parent (say, some value vs null) is a different entity; it has a different key. For example, loading an entity with a null parent, setting the parent to a value, and saving the entity, does not change the entity — it creates a new one. You would still need to delete the old entity and update any foreign key references.

Your best bet is to import the data as-is with the regular ‘member’ field. You can also have the @Parent field (call it anything; you can rename it at any time since it’s not a “real” property). After you migrate, make a pass through your data:

1. Load each Session
2. Check for null parentMember. If null:
3. Assign parentMember and save entity
4. Delete entity with null parentMember

Be very careful of foreign key references if you do this.