Distinct Count via Core Data, NSExpression Into NSFetchedResultsController

Question

Currently using Core Data. I have one table in which I am trying to retrieve information along these lines:

SELECT item, COUNT(*) FROM myTable GROUP BY item;

in order to produce this type of result:

+---------+------+-------+
| item        | COUNT(*) |
+---------+------+-------+
| group 1     |   2      |
| group 2     |   5      |
| group 3     |   8      |
+---------+------+-------+

I had the bright idea to use an NSExpression, with hopes of having Core Data do all the work for me. I am beginning to spin my wheels. The count: expression function is crashing on me. The exception is not very clear. The use of other expression functions, such as sum: doesn't crash the app.

It would be nice to store the results in a NSFetchedResultsController. I've explored other options, none of which aren't too appealing. Would it make more sense to write a SQL query and get it over with, rather than to use Core Data as a SQL wrapper in this case?

For your reference source code is below.

NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"item"];
NSExpression *totalExpression = [NSExpression expressionForFunction:@"count:" arguments:[NSArray arrayWithObject:keyPathExpression]];

NSExpressionDescription * totalExpressionDescription = [[NSExpressionDescription alloc] init];
[totalExpressionDescription setExpression:totalExpression];
[totalExpressionDescription setExpressionResultType:NSInteger64AttributeType];
[totalExpressionDescription setName:@"totalInstances"];

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"myEntity" inManagedObjectContext:self.managedObjectContext];
NSArray *propertiesToFetch = [[NSArray alloc] initWithObjects:strFieldName, totalExpressionDescription, nil];

[fetchRequest setEntity:entity];
[fetchRequest setReturnsDistinctResults:YES];
[fetchRequest setResultType:NSDictionaryResultType];
[fetchRequest setPropertiesToFetch:propertiesToFetch];
[fetchRequest setFetchBatchSize:20];

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:strFieldName ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];

NSFetchedResultsController* aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:[self managedObjectContext] sectionNameKeyPath:nil cacheName:nil];

...

NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error])
{
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
}

Answer 1

This is possible with NSExpression as you originally proposed, in my contrived example I have an entity called "Person" with a property called "emailAddress" that I wish to get the count for.

NSPropertyDescription *propDesc = [[[[model entitiesByName] objectForKey:@"Person"] propertiesByName] objectForKey:@"emailAddress"];
NSExpression *emailExpr = [NSExpression expressionForKeyPath:@"emailAddress"];
NSExpression *countExpr = [NSExpression expressionForFunction:@"count:" arguments:[NSArray arrayWithObject:emailExpr]];
NSExpressionDescription *exprDesc = [[NSExpressionDescription alloc] init];
[exprDesc setExpression:countExpr];
[exprDesc setExpressionResultType:NSInteger64AttributeType];
[exprDesc setName:@"count"];

NSFetchRequest *fr = [NSFetchRequest fetchRequestWithEntityName:@"Person"];
[fr setPropertiesToGroupBy:[NSArray arrayWithObject:propDesc]];
[fr setPropertiesToFetch:[NSArray arrayWithObjects:propDesc, exprDesc, nil]];
[fr setResultType:NSDictionaryResultType];
NSArray *results = [moc executeFetchRequest:fr error:&error];

I wrote a snippet of code to pre-populate the database with 1000 records:

NSArray *emailAddresses = [NSArray arrayWithObjects:@"[email protected]", @"[email protected]", @"[email protected]", @"[email protected]", @"[email protected]", nil];
    for (int i = 0; i < 1000; i++) {
        NSManagedObject *person = [NSEntityDescription insertNewObjectForEntityForName:@"Person" inManagedObjectContext:moc];
        [person setValue:[NSNumber numberWithInt:i] forKey:@"aNumber"];
        [person setValue:[[NSUUID UUID] UUIDString] forKey:@"aUUID"];
        [person setValue:[emailAddresses objectAtIndex:(i % [emailAddresses count])] forKey:@"emailAddress"];
    }

The above code will insert each email address 200 times, here are the results:

2012-05-31 15:17:42.160 Scratch[16084:10d03] CoreData: sql: SELECT t0.ZEMAILADDRESS, COUNT( t0.ZEMAILADDRESS) FROM ZPERSON t0 GROUP BY  t0.ZEMAILADDRESS 
2012-05-31 15:17:42.162 Scratch[16084:10d03] CoreData: annotation: sql connection fetch time: 0.0024s
2012-05-31 15:17:42.163 Scratch[16084:10d03] CoreData: annotation: total fetch execution time: 0.0029s for 5 rows.
(gdb) po results
(NSArray *) $2 = 0x0f811280 <_PFArray 0xf811280>(
{
    count = 200;
    emailAddress = "[email protected]";
},
{
    count = 200;
    emailAddress = "[email protected]";
},
{
    count = 200;
    emailAddress = "[email protected]";
},
{
    count = 200;
    emailAddress = "[email protected]";
},
{
    count = 200;
    emailAddress = "[email protected]";
}
)