Re: CoreData: sorting on a computed attribute
Hi Laurent,
when I faced a similar problem - in my case I needed to filter data acquired
fromCoreData in a TableView for a field value that wasn't part of the model,
but existed as an ivar in the NSManagedObject subclass - I used the following:
[self.sessionController setFilterPredicate:[NSPredicate
predicateWithBlock:^BOOL(id evaluatedObject, NSDictionary *bindings) {
return [(Session*)evaluatedObject fileLocationInvalid];
}]];
Depending on the complexity of the calculation and the impact on performance
you may want to do similar.
You may be able to produce a similar result by using a block as comparator
function for the descriptor. Or write your own comparator function to compare
the calculated values. In both cases you won't need to create a persistent
attribute, but do the calculation in the sort descriptor block or compare
function.
My 2cents,
Volker
Am 14.03.2013 um 18:11 schrieb Laurent Daudelin:
I have a need to sort a CoreData table on one attribute in a table that needs
to be derived from a calculation. I read about Non-Standard Persistent
Attributes and did google and the only way I found to make it work is
according to the following:
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CoreData: sorting on a computed attribute
I have a need to sort a CoreData table on one attribute in a table that needs
to be derived from a calculation. I read about Non-Standard Persistent
Attributes and did google and the only way I found to make it work is
according to the following:
So, suppose I have an entity Entity:
@implementation Entity
@dynamic attribute1;
@dynamic attribute2;
@synthesize positive;
@synthesize negative;
@dynamic net_count;
- (void)setPositive:(NSNumber *)newPositive
{
[self willChangeValueForKey:@positive];
positive = newPositive;
[self didChangeValueForKey:@positive];
[self willChangeValueForKey:@net_count];
self.net_count = [NSNumber numberWithInteger:self.positive.integerValue
- self.negative.integerValue];
[self didChangeValueForKey:@net_count];
}
- (void)setNegative:(NSNumber *)newNegative
{
[self willChangeValueForKey:@negative];
negative = newNegative;
[self didChangeValueForKey:@negative];
[self willChangeValueForKey:@net_count];
self.net_count = [NSNumber numberWithInteger:self.
positive.integerValue - self.negative.integerValue];
[self didChangeValueForKey:@net_count];
}
@end
That seems to work. Records appear in the right order.
Is there anything that I'm doing wrong with the above?
I initially tried to provide a dynamic value when net_count was called, e.g.
implementing an accessor like:
- (NSNumber *)net_count
{
return [NSNumber numberWithInteger:self.positive.integerValue -
self.negative.integerValue];
}
But the records were not ordered properly.
-Laurent.
--
Laurent Daudelin
AIM/iChat/Skype:LaurentDaudelin
http://www.nemesys-soft.com/
Logiciels Nemesys Software
laur...@nemesys-soft.com
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