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 =…

Accepted Answer

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:@"test@test.com", @"1@1.com", @"1@2.com", @"roam@test.com", @"foo@foo.com", 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 = "1@1.com";
},
{
    count = 200;
    emailAddress = "1@2.com";
},
{
    count = 200;
    emailAddress = "foo@foo.com";
},
{
    count = 200;
    emailAddress = "roam@test.com";
},
{
    count = 200;
    emailAddress = "test@test.com";
}
)
```

Distinct Count via Core Data, NSExpression Into NSFetchedResultsController

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