Core Data comparisons within aggregate functions - objective-c

Question

Core Data entity that has two properties on it: startDay and startDateTime. Both properties are NSDate objects. startDay has its time components set to midnight, and startDateTime designates a date with a time. Both “calendar days” within startDay and startDateTime are the same.
Here are some objects
(
{
startDateTime = "2014-05-27 08:00:00 +0000";
startDay = "2014-05-27 00:00:00 +0000";
},
{
startDateTime = "2014-05-27 13:00:00 +0000";
startDay = "2014-05-28 00:00:00 +0000";
}
)
Need to get objects grouped by startDay. Need to know if there is event in the "morning" or "afternoon".
Here's how to get results grouped by startDay and have earliest and latest daily events reported.
- (void)fetchEventDays
{
NSExpression *startDateTimeExpression = [NSExpression expressionForKeyPath:#"startDateTime"];
NSExpression *minStartDateTime = [NSExpression expressionForFunction:#"min:" arguments:[NSArray arrayWithObject:startDateTimeExpression]];
NSExpression *maxStartDateTime = [NSExpression expressionForFunction:#"max:" arguments:[NSArray arrayWithObject:startDateTimeExpression]];
NSExpressionDescription *minStartDateTimeExpression = [[NSExpressionDescription alloc] init];
minStartDateTimeExpression.name = #"minEventStartTime";
minStartDateTimeExpression.expression = minStartDateTime;
minStartDateTimeExpression.expressionResultType = NSDateAttributeType;
NSExpressionDescription *maxStartDateTimeExpression = [[NSExpressionDescription alloc] init];
maxStartDateTimeExpression.name = #"maxEventStartTime";
maxStartDateTimeExpression.expression = maxStartDateTime;
maxStartDateTimeExpression.expressionResultType = NSDateAttributeType;
NSManagedObjectContext *context = [RKObjectManager sharedManager].managedObjectStore.mainQueueManagedObjectContext;
NSEntityDescription* entity = [NSEntityDescription entityForName:#"NIModelScheduleData" inManagedObjectContext:context];
NSAttributeDescription* startDayDesc = [entity.attributesByName objectForKey:#"startDay"];
NSFetchRequest* fetch = [[NSFetchRequest alloc] init];
fetch.entity = entity;
fetch.propertiesToFetch = [NSArray arrayWithObjects:startDayDesc, minStartDateTimeExpression, maxStartDateTimeExpression, nil];
fetch.propertiesToGroupBy = [NSArray arrayWithObject:startDayDesc];
fetch.resultType = NSDictionaryResultType;
NSError *error = nil;
NSArray *results = [context executeFetchRequest:fetch error:&error];
NSLog(#"%#", results);
}
That code is returning
(
{
maxEventStartTime = "2014-05-27 13:00:00 +0000";
minEventStartTime = "2014-05-27 08:00:00 +0000";
startDay = "2014-05-27 00:00:00 +0000";
},
{
maxEventStartTime = "2014-05-28 10:00:00 +0000";
minEventStartTime = "2014-05-28 09:00:00 +0000";
startDay = "2014-05-28 00:00:00 +0000";
}
)
It'd be way cooler if it looked like this
(
{
eveningEvent = YES;
morningEvent = YES;
startDay = "2014-05-27 00:00:00 +0000";
},
{
eveningEvent = NO;
morningEvent = YES;
startDay = "2014-05-28 00:00:00 +0000";
}
)
How can I modify my fetch request to get a date comparison into my NSExpression (or NSExpressionDescription?), so that Core Data checks if min/maxEventStartTime are before/after noon, and returns a BOOL instead of the actual NSDate object.