NSFetchRequest获取属性的最大值。结果每次都不同

Question

NSFetchRequest for an attribute's max value.

But the result turns to a different one every time.

This is my code.

- (NSInteger)getMaxTimeStamp
{
    NSManagedObjectContext *context = [delegate managedObjectContext];


NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Relation" inManagedObjectContext:context]];
[fetchRequest setIncludesPropertyValues:NO]; //only fetch the managedObjectID

NSSortDescriptor *sort = [[NSSortDescriptor alloc]
                          initWithKey:@"ts" ascending:NO];
[fetchRequest setFetchLimit:1];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];

NSError * error = nil;
Relation *relation = [context executeFetchRequest:fetchRequest error:&error].lastObject;

//error handling goes here
if (error) {
    NSLog(@"ERROR: %@",[error localizedDescription]);
}

if (relation.ts) {
    return (NSInteger)relation.ts;
}else {
    return 0;
}

}

and I looped 10 times for result logs

• 2012-07-03 11:05:55.831 Edusorf[1956:15803] max time stamp: 136934464
• 2012-07-03 11:05:55.831 Edusorf[1956:15803] max time stamp: 137170400
• 2012-07-03 11:05:55.832 Edusorf[1956:15803] max time stamp: 137106624
• 2012-07-03 11:05:55.832 Edusorf[1956:15803] max time stamp: 137154096
• 2012-07-03 11:05:55.833 Edusorf[1956:15803] max time stamp: 137107392
• 2012-07-03 11:05:55.833 Edusorf[1956:15803] max time stamp: 137115728
• 2012-07-03 11:05:55.833 Edusorf[1956:15803] max time stamp: 137110608
• 2012-07-03 11:05:55.834 Edusorf[1956:15803] max time stamp: 136828592
• 2012-07-03 11:05:55.834 Edusorf[1956:15803] max time stamp: 136995824
• 2012-07-03 11:05:55.835 Edusorf[1956:15803] max time stamp: 136925072

Could anybody helps me out? thx~~

Answer 1

在数据模型中将ts类型更改为Date。