核心数据谓词，用于查找自日期以来的x天

Question

I have core data model:

Book
----
name 
release_date 

I need to get all books that at least two days passed from their release_date,
And I want to do it with NSPredicate (not to load all of the books to memory, and than loop and check)

What I'm looking for is something like this:

NSDate *now = [NSDate date];
NSPredicate *pred = [NSPredicate predicateWithFormat:@"release_date + "48 hours" <= %@", now];

Answer 1

The predicate will not change the date for you. You should create the appropriate date (based on the current date, and using NSDateComponents ) and pass that in to the predicate. You can use <= in the predicate for filter for older dates.

---

In other words, don't think of the problem as:

@"release_date + "48 hours" <= %@", now

think of it as:

@"release_date <= %@", now - "48 hours"

so you problem is just to get the now - "48 hours" date to pass to the predicate.

Answer 2

I think you want something like that:

NSDate *date2DaysBefore = [NSDate dateWithTimeIntervalSinceNow:-2*24*3600];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"release_date <= %@", date2DaysBefore];