NSExpression

The relatively new NSExpression class is incredibly powerful, yet not really used very often. Part of that is that it's not very well documented. Although the API documentation for NSExpression is fairly well detailed, the listed "companion guide" (Introduction to Predicates Programming) has very little information about how to actually use NSExpression.

NSExpression deserves to be better documented, because it brings to predicate programming (including Core Data), a lot of features from the relational database world that people often complain are missing, like unioning, intersecting, and subtracting resultsets and performing aggregate operations without loading managed objects or faults into memory.

The aggregates functionality is especially important on iOS given the limited memory on most iOS devices. If you've got a large dataset, and you want to get a count of objects, or calculate an average or sum for one of the attributes, you really don't want to have to pull the entire dataset into memory. Even if they're just faults, they're going to eat up memory you don't need to use because the underlying SQLite persistent store can figure that stuff out without the object overhead.

I don't have time to do a full NSExpression tutorial, but I thought it at least worth posting a category on NSManagedObject that lets you take advantage of some of its more useful features.

With this category, to get a sum of the attribute bar on entity Foo, you would do this:

NSNumber *fooSum = [Foo aggregateOperation:@"sum:" onAttribute:@"bar" withPredicate:nil inManagedObjectContext:context];

This will calculate it for you using the database features, NOT by loading all the managed objects into memory. Much more memory and processor efficient than doing it manually.

Cheers. Category follows:

Header File:

@interface NSManagedObject(MCAggregate)
+(NSNumber *)aggregateOperation:(NSString *)function onAttribute:(NSString *)attributeName withPredicate:(NSPredicate *)predicate inManagedObjectContext:(NSManagedObjectContext *)context
@end

Implementation File:

+(NSNumber *)aggregateOperation:(NSString *)function onAttribute:(NSString *)attributeName withPredicate:(NSPredicate *)predicate inManagedObjectContext:(NSManagedObjectContext *)context
{
    NSExpression *ex = [NSExpression expressionForFunction:function 
        arguments:[NSArray arrayWithObject:[NSExpression expressionForKeyPath:attributeName]]];

    NSExpressionDescription *ed = [[NSExpressionDescription alloc] init];
    [ed setName:@"result"];
    [ed setExpression:ex];
    [ed setExpressionResultType:NSInteger64AttributeType];

    NSArray *properties = [NSArray arrayWithObject:ed];
    [ed release];

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    [request setPropertiesToFetch:properties];
    [request setResultType:NSDictionaryResultType];

    if (predicate != nil)
        [request setPredicate:predicate];

    NSEntityDescription *entity = [NSEntityDescription entityForName:[self className] 
        inManagedObjectContext:context];
    [request setEntity:entity];

    NSArray *results = [context executeFetchRequest:request error:nil];
    NSDictionary *resultsDictionary = [results objectAtIndex:0];
    NSNumber *resultValue = [resultsDictionary objectForKey:@"result"];
    return resultValue;

}
@end