Return most recent NSManagedObjects only from NSFetchRequest - ios

Question

I am using an NSFetchedResultsController and need to return distinct objects based on their latest dates and name. Each date is stored as an NSDate.
Example:
Object1
name:Object1
date:01/01/2001
Object2
name:Object
date:01/02/2001
Object3
name:OtherObject
date:01/10/2001
Object4
name:OtherObject
date:02/01/2001
Expected results (Return only the latest date objects when a duplicate is found):
name:Object
date:01/02/2001
name:OtherObject
date:02/01/2001
// Below returns everything sorted correctly although I only need the latest of each match based on the date.
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription
entityForName:#"Object" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
// Create the sort descriptors array.
NSSortDescriptor *name = [[NSSortDescriptor alloc] initWithKey:#"name" ascending:YES];
NSSortDescriptor *date = [[NSSortDescriptor alloc] initWithKey:#"date" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:name, date, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
EDIT
As of now I am fetching all of the objects into an array, for each object in array1 I cycle thru and add / compare it to the latest matching objects in array2 with the same name and remove the oldest date entry from array2 then I'm left with unique latest dates in array2. I was hoping there was a less cumbersome way to do this with a single fetchrequest / subquery.

Answer 1

Core Data doesn't automatically track insertion order. When you get an fetched object back, its entities will be in any random order.
My immediate thoughts on this are that you need another attribute to track the insertion time and reorder/sort by this attribute.
So, your attribute would be need to be a date time with seconds. You would call this:
NSDate *now = [[NSDate alloc] init];
and store it in the attribute. When you fetch the object, sort by the key of this attribute.

Answer 2

Try this...
Create an NSSet of the attribute name to return a unique set of data.
Fetch the latest attribute date for each attribute name.
For example:
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Object"
inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
NSArray *objects = [self.managedObjectContext executeFetchRequest:fetchRequest];
NSMutableArray *arrayObjectNames = [[NSMutableArray alloc] init];
for (Object *object in objects) {
[arrayObjectNames addObject:object.name];
}
NSSet *setObjectNames = [NSSet setWithArray:arrayObjectNames];
NSString *key = #"name";
NSMutableDictionary *dictNamesDates = [NSMutableDictionary dictionary];
for (NSString *objectName in setObjectNames) {
NSFetchRequest *fetchName = [[NSFetchRequest alloc] initWithEntityName:#"Object"];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"%K == %#", key, objectName];
[fetchName setPredicate:predicate];
NSSortDescriptor *sortDate = [[NSSortDescriptor alloc] initWithKey:#"date" ascending:YES];
[fetchName setSortDescriptors:#[sortDate]];
Object *targetObject = [[self.managedObjectContext executeFetchRequest:fetchName] lastObject];
if (targetObject) [dictNamesDates setObject:targetObject.date forKey:objectName];
//OR as an alternative but achieving the same outcome...
if (targetObject) [dictNamesDates setValue:targetObject.date forKey:targetObject.name];
}
This should create an NSMutableDictionary that contains each attribute name as the key and the corresponding latest attribute date as the value.
Does this help?