Google App Engine DataStore - 如何以有效的方式從Java中的子表的鍵中選擇父實體？

Question

我試圖從表示 Building Scalable, Complex Apps on App Engine翻譯這個Python代碼到Java，

是否有支持GAEJ做到這一點？

indexes = db.GqlQuery("SELECT __key__ FROM MessageIndex " "WHERE receivers = :1", me) 
keys = [k.parent() for k in indexes] 
messages = db.get(keys) 

下面我在Java中實現。這些功能和性能是否相當？如果我有大量的密鑰會發生什麼？

Query query1 = new Query("MessageIndex"); 
query1.setKeysOnly(); 
query1.addFilter("receivers", FilterOperator.EQUAL, me); 

PreparedQuery pq2 = datastore.prepare(query1); 

Iterable<Entity> entities = pq2.asIterable(); 
HashSet<Key> keys = new HashSet<Key>(); 
    for (Entity en : entities) 
     keys.add(en.getParent()); 

Query query2 = new Query("Messages"); 
query2.addFilter("__key__", FilterOperator.IN, keys); 

Answer 1

看僅有一名DatastoreService API https://developers.google.com/appengine/docs/java/javadoc/com/google/appengine/api/datastore/DatastoreService

DatastoreService.get（鍵）比用鍵查詢更有效。

Answer 2

正如@Rick已經指出的那樣，獲取速度比查詢要快得多。你只需要改變你的代碼的最後一部分。相反的：

Query query2 = new Query("Messages"); 
query2.addFilter("__key__", FilterOperator.IN, keys); 

只是做：

Map<Key,Entity> messages = datastore.get(keys);