Maximum value for attribute in Core Data

+ (int32_t)maximumPositionInContext:(NSManagedObjectContext *)context {
 // sample using attribute named "position" on entity "Item"

 NSFetchRequest *request = [NSFetchRequest new];
 request.entity = [NSEntityDescription entityForName:@"Item" inManagedObjectContext:context];
 request.resultType = NSDictionaryResultType;
 // technically not necessary to set fetchLimit = 1, resultType could be (Item *) managed object

 NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"position"];
 NSExpression *maxExpression = [NSExpression expressionForFunction:@"max:" arguments: @[ keyPathExpression ]];
 NSExpressionDescription *expression = [NSExpressionDescription new];
 expression.name = @"maxPosition";
 expression.expression = maxExpression;

 // This changes the return value in valueForKey: below
 expression.expressionResultType = NSInteger32AttributeType;

 request.propertiesToFetch = @[ expression ];

 // Execute the fetch.
 NSError *error = nil;
 NSArray *objects = [context executeFetchRequest:request error:&error];
 if (objects && objects.count) {
 return [[objects[0] valueForKey:@"maxPosition"] integerValue];
 }

 return -1;
}