使用 objectify 如何获取对象的属性子集
我有一个使用 objectify 存储的大对象。我需要这些对象的列表，其中仅填充了属性的子集。这怎么能做到呢？
I have a large object that I store using objectify. I need a list of those objects with only subset of the properties populated. How can this be done?
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App Engine 将实体存储和检索为编码的协议缓冲区。底层基础设施无法仅存储、更新或检索实体的一部分，因此拥有一个库来执行此操作是没有意义的 - 因此，Objectify 与其他库一样，没有这样做。如果您经常需要仅访问实体的一部分，请将这些字段拆分为单独的实体。
App Engine stores and retrieves entities as encoded Protocol Buffers. There's no way for the underlying infrastructure to store, update, or retrieve only part of an entity, so there's no point having a library that does this - hence Objectify, like other libraries, don't. If you regularly need to access only part of an entity, split those fields into a separate entity.
在 noSql 数据库中将实体一分为二并不是一个好主意：当您需要读取条目列表时，您将不得不执行 n 个请求来获取列表的第二部分（如果您的数据被拆分为 nxm更多实体）。这自然是因为 noSql 数据库中不可能有 join。
可以做的是“缓存”：在另一个实体中复制所需的子集以获得最大的性能。它的缺点是必须在主实体的持久性上写入两次（如果子集的字段发生更改）。
我通常做的是在需要读取子集并在需要更高性能时返回到它的类上编写
/** OPTIMIZE xxxx */注释。It's not a good idea to split an entity in two in a noSql database: when you need to read a list of entries, you would be obliged to do n requests to get the second part of the list (n x m if your data is split in more entities). This is naturally due to the fact that there is no possible join in noSql databases.
What could be done is to "cache": duplicate the needed subset in another entity to get the most of performance. It has the disadvantage of being obliged to write twice on a persist of the main entity (if a field of the subset was changed).
What I usually do is write a
/** OPTIMIZE xxxx */comment on the class that needs to read a subset and get back to it when I need more performance.