how to set Custom attribute of NSManagedObject which is calculated from other attributes ?

Question

I am using core data framework to manage objects.i have an entity which has several attributes of decimal types. Among them is attribute which is mathematically calculated from other attributes. Ex :-

@interface Marks : NSManagedObject
{
}

@property (nonatomic, retain) NSDecimalNumber * answerGradeA;
@property (nonatomic, retain) NSDecimalNumber * answerGradeB;
@property (nonatomic, retain) NSDecimalNumber * answerGradeC;
@property (nonatomic, retain) NSDecimalNumber * total;

Here i want attribute total = 3xanswerGradeA + 2xanswerGradeB + 1xanswerGradeC

if it is possible to do like this, then how ? please reply. Thanks in advance.

Answer 1

Why not make it a category and compile it in a separate file? (Strictly speaking, total should not be part of CoreData.)

@interface Marks (Calculated)
@property (nonatomic, readonly) NSDecimalNumber* total;
@end

@implementation Marks (Calculated)
- (NSDecimalNumber*) total { 
  return whatEverYouLike; 
}
@end

Answer 2

The Core Data way is to add 'total' as an attribue to the model and mark it 'transient'. You then provide the implementation in a subclass.

@interface Marks :  NSManagedObject  
{
}
@property (nonatomic, readonly) NSDecimalNumber* total;
@end

@implementation Marks (Calculated)
- (NSDecimalNumber*) total { 
    return (3 * [self valueForKey:@"answerGradeA"]) + (2 * [self valueForKey:@"answerGradeB"]) + [self valueForKey:@"answerGradeC"]; 
}
+ (NSSet *)keyPathsForValuesAffectingTotal
{
    return [NSSet setWithObjects:@"answerGradeA", @"answerGradeB", @"answerGradeC", nil];
}
@end

This will ensure proper caching and updating of total.

Answer 3

I want to post a little modification to Looji's answer.

@interface Marks :  NSManagedObject  
{
}
@property (nonatomic, retain) NSDecimalNumber * answerGradeA;
@property (nonatomic, retain) NSDecimalNumber * answerGradeB;
@property (nonatomic, retain) NSDecimalNumber * answerGradeC;
@property (nonatomic, readonly) NSDecimalNumber* total;
@end

@implementation Marks (Calculated)
- (NSDecimalNumber*) total { 
return (3 * [self valueForKey:@"answerGradeA"]) + (2 * [self valueForKey:@"answerGradeB"]) + [self valueForKey:@"answerGradeC"]; 
}
+ (NSSet *)keyPathsForValuesAffectingTotal
{
return [NSSet setWithObjects:@"answerGradeA", @"answerGradeB", @"answerGradeC", nil];
}
@end