I’ve noticed that AppEngine does not populate Key fields by default when querying. I’m sure it’s documented, but I can’t remember where. At any rate, you can annotate with defaultFetchGroup to resolve this. Same goes for properties of type Text and Blob.
...
import com.google.appengine.api.datastore.Key;
@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class PrayerItem implements Serializable
{
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Long id;
@Persistent(defaultFetchGroup="true")
private Key list;
...
}
In the case of owned relationships, where a property is represented by its actual domain class rather than a Key, it is also necessary to use defaultFetchGroup=”true”. Even though owned properties are in the same entity group and are therefore eligible for inclusion in a transaction, the AppEngine Datastore doesn’t allow joins, so owned relationships are not returned in the initial query. When you set defaultFetchGroup=”true” on an owned property, you will get this warning:
WARNING: Meta-data warning for com…your_owned_property_name: The datastore does not support joins and therefore cannot honor requests to place child objects in the default fetch group. The field will be fetched lazily on first access. You can modify this warning by setting the datanucleus.appengine.ignorableMetaDataBehavior property in your config. A value of NONE will silence the warning. A value of ERROR will turn the warning into an exception.
Lazy fetching does work as described in the message.
