NSFetchRequest predicate with dates within the last 24 hours?

开发者_运维问答

I have a Core Data entity named RSSItem with a Date attribute named publicationDate.

Is there a way to create a predicate for a NSFetchRequest that would include all entities with a publication date in the last 24 hours?

I have tried this:

NSString* expression = @"publicationDate.timeIntervalSinceNow >= -86400";
return [NSPredicate predicateWithFormat:expression];

But that yields a nil predicate, thus matching everything.

I would like to have a live predicate that change over time. Therefor a predicate with a NSDate instance is not possible, since it defines an immutable single point in time, not a relative time such as the last 24 hours.

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I think this should work:

[NSPredicate predicateWithFormat:@"publicationDate >= %@", [NSDate dateWithTimeIntervalSinceNow:-86400];

Update

You could try an alternate approach to get the live predicate to work. I think that you have to update the attribute(s) for an entity that are used in a predicate to trigger a filtering anyway so even if your proposed predicate was possible you would still have to loop through all items and fake an update for publicationDate.

What you could try is add a bool attribute shouldDisplay to RSSItem and a method `updateShouldDisplay'

- (void)updateShouldDisplay {
    self.shouldDisplay = [NSNumber numberWithBool:[self.publicationDate compare:[NSDate dateWithTimeIntervalSinceNow:-86400]] == NSOrderedDescending];
}

Then whenever you are refreshing the RSS feed you loop through all entities and call updateShouldDisplay. You probably have to call updateShouldDisplay from awakeFromFetch as well.

And just to be complete the predicate would look like:

[NSPredicate predicateWithFormat:@"shouldDisplay == YES"]

I'm not 100% on this but I think you can't use transient properties in an NSPredicate which means you have to store shouldDisplay.

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• Relative date is not a problem. You don't need the predicate to change. The 'last 24 hour' boundary is a fixed point in time relative to the current time. Create an NSDate and stick it in the predicate (like in Robert's answer above)

• For "live-ness", you could use a fetched property with the above predicate, putting the cutoff date/time in the userInfo dictionary. The results are cached by CoreData (because true live properties would cause a DB query each time, and could be expensive) so you need to call NSManagedObjectContext refreshObject: to update the data (say, whenever you're finished inserting a bunch of new RSSItems after a refresh).