核心数据组NSDate的月份/年份 [英] Core Data group NSDate into month/year

问题描述

假设我在表中有一些NSDates：

Suppose I have some NSDates in a table:

• 2012年1月11日


• 1/16/2012


• 2/14/2012


• 4/13/2012


• 4 / 14/2012


• 2012/4/18

• 1/11/2012
• 1/16/2012
• 2/14/2012
• 4/13/2012
• 4/14/2012
• 4/18/2012

我想将这些日期分组这样的月份和年份：

I would like to group these dates by month and year such that I get:

• 1/2012


• 2/2012


• 4/2012

我目前求助于获取表中的每个记录，并执行 valueForKeyPath 在日期列上，提取 NSDateComponents 并将结果存储在 NSSet 。这似乎效率很低。

I currently resort to fetching every record in the table, performing a valueForKeyPath on the date column, extracting NSDateComponents and storing the results in an NSSet. This seems a bit inefficient.

我想知道是否有一种方法可以编写NSPredicate来将这些日期分组在数据库端。

I would like to know if there is a way to write an NSPredicate that will group these dates on the database side.

推荐答案

我做类似的事情，并且我在名为 sortDate 。
我从自定义日期设置器设置了sortDate。

I do something similar, and I use an extra attribute in the entity that is named sortDate. I set the sortDate from my custom date setter.

类似这样的东西：

- (NSDate *)monthDateFromDate:(NSDate *)date {
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    NSDateComponents *components = [calendar components:NSYearCalendarUnit|NSMonthCalendarUnit fromDate:date];
    NSDate *monthDate = [calendar dateFromComponents:components];
    return monthDate;
}

- (void)setDate:(NSDate *)date {
    [self willChangeValueForKey:@"date"];
    [self setPrimitiveStartDate:date];
    [self didChangeValueForKey:@"date"];

    NSDate *sortDate = [self monthDateFromDate:date];
    [self willChangeValueForKey:@"sortDate"];
    [self setPrimitiveSortDate:sortDate];
    [self didChangeValueForKey:@"sortDate"];
}

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