NSFetchRequest按月分组 [英] NSFetchRequest group by month

问题描述

在我的模型中，我有一个Sale实体，每个Sale实体都有一个date属性（NSDate）。
我想计算每个月的销售额。

In my model, I have a Sale entity, each Sale entity has a date property (NSDate). I want to compute the sum of sales for each month.

我知道如何创建用于计算总和的表达式，我想找出的是

I know how to create the expression for computing the sum, what I want to find out is how do I tell NSFetchRequest to group by month.

推荐答案

您可以使用 NSFetchedResultsController 为您进行分组。它在工厂方法中有一个名为 sectionNameKeyPath 的参数，可用于分隔数据。您将免费获得所有 NSIndexPath 算法，以及无数的优化。

You can use a NSFetchedResultsController to do the grouping for you. It has a parameter in the factory method called sectionNameKeyPath which you can use to separate your data. You will get all the NSIndexPath arithmetic for free, and a myriad of optimizations as well.

由于日期实际上是连续的，因此您必须使用瞬时属性，甚至必须保留月份数据（这很便宜，所以没关系）。看看Apple的示例代码，其中的节标题中带有日期，然后类似地进行。

Because dates are practically continuous, you have to work with transient properties, or even persist the month data (which is cheap, so it does not matter). Take a look at Apple's sample code with dates in section headers and proceed analogously.

Apple示例代码： DateSectionTitles

特别注意 APLEvent的实现。

Apple Sample Code: DateSectionTitles
Pay particular attention to the implementation of APLEvent.

这篇关于NSFetchRequest按月分组的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！