子數據存儲區實體的鍵返回爲空

Question

我有一個典型的人地址實體關係。在查詢Person的數據存儲之後，我查詢Person作爲地址的密鑰。密鑰（即addrKey，見下文）始終返回爲空。但是我在數據存儲中查看，我看到了Person和Address實體以及它們的鍵。所以很明顯，Key addrKey = (Key) person.getProperty("address")行並沒有做我認爲應該做的事情。任何想法如何解決這一問題？

DatastoreService datastore = DatastoreServiceFactory.getDatastoreService(); 
    Map<Key, Entity> entities = datastore.get(keys); 

    List<Person> result = new ArrayList<Person>(); 
    Iterator<Entry<Key, Entity>> it = entities.entrySet().iterator(); 
    while (it.hasNext()) { 
     Map.Entry<Key, Entity> ent = it.next(); 

     final Entity person = ent.getValue(); 
     Key key = person.getKey(); 
     name = (Long) person.getProperty("name"); 
     Address address = getAddress(datastore, person); 

... 
    } 


private Address getAddress(DatastoreService datastore, Entity person) { 
    Key addrKey = (Key) person.getProperty("address"); 
    try { 
     Entity d = datastore.get(addrKey); 
     String street = (String) d.getProperty("street"); 
… 
} 

Answer 1

我的猜測是你堅持地址和人在同一時間。但是，默認情況下，地址默認情況下的密鑰是不完整的（假設您依靠密鑰生成自動ID）。因此，您親自存儲了一個不完整的密鑰。並且使用不完整的鍵提取不起作用。

我寫了一個快速測試案例：

import com.google.appengine.api.datastore.DatastoreService; 
import com.google.appengine.api.datastore.DatastoreServiceFactory; 
import com.google.appengine.api.datastore.Entity; 
import com.google.appengine.api.datastore.Key; 

public class TempTest extends PersistenceTestHelper { 

    @Test 
    public void testStackOverflow() throws Exception { 

    Entity address = new Entity("address", 1); 
    address.setProperty("street", "my street"); 

    Entity person = new Entity("person", 1); 
    person.setProperty("addressKey", address.getKey()); 
    System.out.println(person); 

    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService(); 
    datastore.put(person); 
    datastore.put(address); 

    Entity personFromDatastore = datastore.get(person.getKey()); 
    Key addressKey = (Key) personFromDatastore.getProperty("addressKey"); 
    Entity addressFromDatastore = datastore.get(addressKey); 
    System.out.println("fetched street: " + addressFromDatastore.getProperty("street")); 

    address = new Entity("address"); 
    System.out.println("Address with incomplete key: " + address.getKey()); 
    try { 
     addressFromDatastore = datastore.get(address.getKey()); 
    } catch (IllegalArgumentException e) { 
     System.out.println(e); 
    } 
    } 

} 

這是輸出：

<Entity [person(1)]: 
    addressKey = address(1) 
> 
fetched street: my street 
Address with incomplete key: address(no-id-yet) 
java.lang.IllegalArgumentException: address(no-id-yet) is incomplete. 

如果我錯了，這將是很好理解爲什麼你有一個嘗試catch在你的例子中。而且在預先放置時間和pos-get時間的Person實體和Address實體的打印輸出將會很好。