在低级 API 中生成 id
我必须使用低级 API 在 Google App Engine 中保留 Value 类型的实体。我一直在搜索，但只找到了这样的示例：
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Key k = KeyFactory.createKey(Value.class.getSimpleName(), id);
Entity entity = new Entity(k);
entity.setProperty("column1", value.getColumn1());
entity.setProperty("column2", value.getColumn2());
datastore.put(entity);
我的问题是我事先不知道 id （值的标识符），因为我需要将它生成为序列。这将是在低级 API 中执行此操作的方法，就像在 JDO 中执行的那样：
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Long id;
如何在低级中检索 id 或将其配置为作为序列生成？
谢谢。
I have to use the low level API to persist an entity of type Value in Google App Engine. I've been searching and I have only found a examples in this way:
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Key k = KeyFactory.createKey(Value.class.getSimpleName(), id);
Entity entity = new Entity(k);
entity.setProperty("column1", value.getColumn1());
entity.setProperty("column2", value.getColumn2());
datastore.put(entity);
My problem is that I don't know the id (identifier of Value) in advance because I need it to be generated as a sequence. It would be the way to do it in the low level API as it is done in JDO as:
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Long id;
How can I retrieve the id in the low level or configure it to be generated as a sequence?
Thanks.
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Entity类有许多构造函数。使用采用单个字符串（种类名称）的字符串，当您将其存储在数据存储中时，将为您生成 ID。The
Entityclass has many constructors. Use the one that takes a single string - the kind name - and the ID will be generated for you when you store it in the datastore.也许尝试使用“allocateIds”来分配一系列要使用的 Id？这将为您提供一组可供使用的保留密钥。我怀疑您是否能够获得严格的序列，例如在关系数据库中，但至少您将能够获得有保证的唯一且可用的密钥。
请参阅 DatastoreService 的文档：
http://code.google.com/appengine/docs/java/javadoc/com/google/appengine/api/datastore/DatastoreService.html#allocateIds%28com.google.appengine.api.datastore。 Key,%20java.lang.String,%20long%29
另外，为了获得进一步的指导，您可以查看 Datanucleus 如何使用此 API：
http://code.google.com/p/datanucleus-appengine/source/browse/trunk/src/org/datanucleus/store/appengine/valuegenerator/SequenceGenerator.java?r=473
Perhaps try to use "allocateIds" to allocate a range of Ids to use? This will give you a set of reserved keys to use. I doubt you will be able to get a strict sequence such as in relational databases, but at least you would be able to get guaranteed unique and usable keys.
See the documentation for DatastoreService:
http://code.google.com/appengine/docs/java/javadoc/com/google/appengine/api/datastore/DatastoreService.html#allocateIds%28com.google.appengine.api.datastore.Key,%20java.lang.String,%20long%29
Also for further guidance you might take a look at how Datanucleus uses this API:
http://code.google.com/p/datanucleus-appengine/source/browse/trunk/src/org/datanucleus/store/appengine/valuegenerator/SequenceGenerator.java?r=473