Question

In this example, the Google App Engine documentation makes the Customer the entity group parent of the AccountInfo entity. Wouldn’t AccountInfo encapsulate Customer rather than the other way around? Normally I would think of an AccountInfo class as including all of the information about the Customer.

import javax.jdo.annotations.IdGeneratorStrategy;
import javax.jdo.annotations.PersistenceCapable;
import javax.jdo.annotations.Persistent;
import javax.jdo.annotations.PrimaryKey;
import com.google.appengine.api.datastore.Key;
import com.google.appengine.api.datastore.KeyFactory;

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

    public void setKey(Key key) {
        this.key = key;
    }
}

// ...
        KeyFactory.Builder keyBuilder = new KeyFactory
         .Builder(Customer.class.getSimpleName(), "custid985135");

        keyBuilder.addChild(AccountInfo.class.getSimpleName(), "acctidX142516");

        Key key = keyBuilder.getKey();

        AccountInfo acct = new AccountInfo();
        acct.setKey(key);
        pm.makePersistent(acct);

Answer 1

In real life a Customer can have more than one Account, for instance a current account and a savings account. Customers which are organisations rather than individuals may have several accounts for various different purposes.

Consequently, it makes perfect sense to have a single Customer entity which can own one or more AccountInfo entities.