an NSPredicate for selecting a row with maximum value by group - objective-c

Question

I'm using Core Data to store entities in the form
TrackerEntry
username
timestamp
I want to select the latest record for each user. Using sql it would be something like
SELECT MAX(timestamp) FROM Log GROUP BY username
Is there anyway to create an NSPredicate to do this?

Answer 1

I would do it using NSExpression. This bit of code below won't work for you because you will have to group by username too, but it's a start for the best way of doing this without having to fetch everything. You want to perform the max and group in the db, not in memory - as it will be faster:
NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
// Expression for the max
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:#"timestamp"];
NSEntityDescription *entity = [NSEntityDescription entityForName:entityName
inManagedObjectContext:self.coreDataStack.managedObjectContext];
[fetchRequest setEntity:entity];
[fetchRequest setResultType:NSDictionaryResultType];
NSExpression *valueSumExpression = [NSExpression expressionForFunction:#"max:" arguments:[NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *expressionDescription = [[[NSExpressionDescription alloc] init] autorelease];
[expressionDescription setName:#"maxTimestamp"];
[expressionDescription setExpression:valueSumExpression];
[expressionDescription setExpressionResultType:NSDecimalAttributeType];
[fetchRequest setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];
// Filter
//NSPredicate *pred = [NSPredicate predicateWithFormat:#" your predicate here"];
//fetchRequest.predicate = pred;
NSArray *results = [self.coreDataStack.managedObjectContext executeFetchRequest:fetchRequest error:nil];
if([results count] == 0) {
} else {
// [[results objectAtIndex:0] valueForKey:#"maxTimestamp"];
}

Answer 2

This worked for me, but it is a loop. First you have to get the unique names into an array.
NSFetchRequest *nameRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Log" inManagedObjectContext:self.managedObjectContext];
nameRequest.entity = entity;
nameRequest.resultType = NSDictionaryResultType;
[nameRequest setPropertiesToFetch:[NSArray arrayWithObject:[[entity propertiesByName] objectForKey:#"name"]]];
NSArray *allNames = [self.managedObjectContext executeFetchRequest:nameRequest error:nil];
names = [allNames valueForKeyPath:#"#distinctUnionOfObjects.name"];
[nameRequest release];
NSLog(#"%#", names);
Not very efficient or convenient. After all, Core Data is not a database but an object graph.
Then you can loop through these and fetch just the top one.
NSMutableArray *mutableResults = [NSMutableArray array];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = [NSEntityDescription entityForName:#"Log" inManagedObjectContext:self.managedObjectContext];
NSSortDescriptor *numberSort = [NSSortDescriptor sortDescriptorWithKey:#"number" ascending:NO];
request.sortDescriptors = [NSArray arrayWithObject:numberSort];
request.fetchLimit = 1;
for (NSString *s in names) {
NSPredicate *pred = [NSPredicate predicateWithFormat:#"name = %#", s];
request.predicate = pred;
NSArray *results = [self.managedObjectContext executeFetchRequest:request error:nil];
[mutableResults addObject:[results objectAtIndex:0]];
}
[request release];
NSLog(#"%#", mutableResults);