一对多关系。从数据存储中选择对象
我省略了一些代码（包声明、导入、其他字段） 为了简短。 我这里有简单的一对多关系。 直到这一刻为止一切都很好。
@PersistenceCapable(identityType = IdentityType.APPLICATION,
detachable="true")
class Restaurant implements Serializable {
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
Key id
@Persistent(mappedBy = "restaurant")
List<RestaurantAddress> addresses = new ArrayList<RestaurantAddress>()
}
//-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
@PersistenceCapable(identityType = IdentityType.APPLICATION,
detachable="true")
class RestaurantAddress implements Serializable {
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
Key id
@Persistent
Restaurant restaurant
}
现在我需要从数据库获取（选择）所有餐厅：
def getRestaurantsToExport(final String dst, final int count) {
String field = restaurantExportFields[dst]
return transactionExecute() { PersistenceManager pm ->
Query q = pm.newQuery(Restaurant.class)
q.filter = "$field == null"
q.setRange(0, count)
return q.execute()
}
}
但是有有问题 - 查询给了我 12 家餐厅（如 DB 中所示）但是 每个餐厅都有 0 个地址，但在数据存储中每个餐厅都有 至少 2 个地址。
有没有人遇到同样的问题或知道解决方案？
I've omitted some code(package declarations, imports, other fields)
for shortness.
I have here simple One-to-Many relation.
It worked fine till this moment.
@PersistenceCapable(identityType = IdentityType.APPLICATION,
detachable="true")
class Restaurant implements Serializable {
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
Key id
@Persistent(mappedBy = "restaurant")
List<RestaurantAddress> addresses = new ArrayList<RestaurantAddress>()
}
//-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
@PersistenceCapable(identityType = IdentityType.APPLICATION,
detachable="true")
class RestaurantAddress implements Serializable {
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
Key id
@Persistent
Restaurant restaurant
}
Now i need to get(select) all the Restaurants from DB:
def getRestaurantsToExport(final String dst, final int count) {
String field = restaurantExportFields[dst]
return transactionExecute() { PersistenceManager pm ->
Query q = pm.newQuery(Restaurant.class)
q.filter = "$field == null"
q.setRange(0, count)
return q.execute()
}
}
But there are on problem - query gives me 12 restaurants(as in DB) but
every Restaurant has 0 Address but in Datastore every Restaurant has
minimum 2 addresses.
Have anyone the same problem or knows the solution ?
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评论（2）
您确定地址没有延迟加载吗？只是猜测......是否有某种方法可以强制“急切”加载对象
are you sure the Addresses are not lazy loaded? Just a guess... is there some way to force an "eager" loading of the objects
如果有人遇到同样的问题：
替换
为
另一种方法是你必须“触摸”地址属性
关闭前检索到的列表中的每个餐厅
持久管理器。 PersistenManager 关闭后，您不能
从数据存储中检索任何内容，而 Restaurant 保持为空。
在 google-appengine-java 用户的帮助下找到了解决方案。
If someone will have the same problem:
Replace
with
Another method is that you have to "touch" addresses property for
every Restaurant in the retrieved list before closing
PersistentManager. After PersistenManager being closed you cannot
retrieve anything from datastore and Restaurant keeps null.
Solution found with help of google-appengine-java users.