如何根据多个Core Data属性中的计算值进行排序 [英] How to sort based on a calculated value from multiple Core Data attributes

问题描述

我试图为核心数据获取请求创建一个排序描述符，以获取按计算值排序的对象（例如，两个或多个属性的总和）。这可能吗？提前感谢。

I'm trying to create a sort descriptor, for a core data fetch request, that will fetch objects that are ordered by a calculated value (e.g., the sum of two or more attributes). Is this possible? Thanks in advance.

推荐答案

从核心数据编程指南：

您不能使用基于瞬态属性的谓词获取
（虽然您可以使用瞬态属性在内存中过滤
）。 ...到
总结，但是，如果你直接执行fetch，你应该
通常不添加基于Objective-C的谓词或排序描述符到
获取请求。

You cannot fetch using a predicate based on transient properties (although you can use transient properties to filter in memory yourself). ... To summarize, though, if you execute a fetch directly, you should typically not add Objective-C-based predicates or sort descriptors to the fetch request. Instead you should apply these to the results of the fetch.

因此，按计算值排序似乎不可能。您必须预先计算排序值，并将其作为（非瞬态）属性存储在对象中。

So sorting by a calculated value seems not to be possible. You have to pre-calculate the sorting value and store it as (non-transient) attribute in your objects.

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