Cálculo del valor máximo para un atributo de datos centrales - NSCFNumber error

Question

Necesito averiguar el valor máximo de un atributo de una entidad de datos central.

Todavía estoy firmemente en la curva de aprendizaje de Cocoa, y esta es una aplicación de prueba simple que estoy usando para aprender.

La aplicación importa fortunas de un archivo de texto y muestra una tabla en la pantalla. Las importaciones se realizan en un hilo de fondo separado.

me encontré con este código en línea, que he tratado de conseguir trabajo:

- (double)getMaxID 
{ 
    NSLog(@"in getMaxID"); // debug 

    // Use a new moc with the original persistentStoreCoordinator to ensure thread safety 
    NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] init]; 
    [moc setPersistentStoreCoordinator:[[self delegate] persistentStoreCoordinator]]; 

    // Create fetch 
    NSFetchRequest *fetch = [[NSFetchRequest alloc] init]; 
    [fetch setEntity:[NSEntityDescription entityForName:@"Fortune" inManagedObjectContext:moc]]; 
    [fetch setResultType:NSDictionaryResultType]; 

    // Expression for Max ID 
    NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"id"]; 
    NSExpression *minExpression = [NSExpression expressionForFunction:@"max:" arguments: [NSArray arrayWithObject:keyPathExpression]]; 
    NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init]; 
    [expressionDescription setName:@"maxID"]; 
    [expressionDescription setExpression:minExpression]; 
    [expressionDescription setExpressionResultType:NSDoubleAttributeType]; 
    [fetch setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]]; 

    // Execute the fetch. 
    double theID = 0; 
    NSError *error = nil; 
    NSArray *objects = nil; 
    objects = [moc executeFetchRequest:fetch error:&error]; // crashes here 

    if (objects && ([objects count] > 0)) 
    { 
     NSLog(@"query successful"); // debug 
     theID = [((NSNumber *)[[objects objectAtIndex:0] valueForKey:@"maxID"]) doubleValue]; 
    } 
    else 
    { 
     NSLog(@"Setting default value for theID"); // debug 
     theID = 0; 
    } 

    return(theID); 
} 

Mi entidad es llamada "Fortuna" y el atributo se llama "id" (un doble).

Cuando se ejecuta el código, falla cuando se ejecuta la solicitud de búsqueda. La consola muestra esto:

 
2009-12-18 00:53:42.777 FortunesHelperMVC[4027:1703] -[NSCFNumber count]: unrecognized selector sent to instance 0x1004d7b10 
2009-12-18 00:53:42.778 FortunesHelperMVC[4027:1703] An uncaught exception was raised 
2009-12-18 00:53:42.778 FortunesHelperMVC[4027:1703] -[NSCFNumber count]: unrecognized selector sent to instance 0x1004d7b10 
2009-12-18 00:53:42.779 FortunesHelperMVC[4027:1703] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSCFNumber count]: unrecognized selector sent to instance 0x1004d7b10' 
*** Call stack at first throw: 
(
0 CoreFoundation 0x00007fff83ed9444 __exceptionPreprocess + 180 
1 libobjc.A.dylib 0x00007fff85fbb0f3 objc_exception_throw + 45 
2 CoreFoundation 0x00007fff83f321c0 +[NSObject(NSObject) doesNotRecognizeSelector:] + 0 
3 CoreFoundation 0x00007fff83eac08f ___forwarding___ + 751 
4 CoreFoundation 0x00007fff83ea81d8 _CF_forwarding_prep_0 + 232 
5 Foundation 0x00007fff88a5609e +[_NSPredicateUtilities max:] + 46 
6 Foundation 0x00007fff8893ce72 -[NSFunctionExpression expressionValueWithObject:context:] + 530 
7 CoreData 0x00007fff8613b5b1 -[NSMappedObjectStore executeFetchRequest:withContext:] + 2081 
8 CoreData 0x00007fff8613ad10 -[NSMappedObjectStore executeRequest:withContext:] + 80 
9 CoreData 0x00007fff86108900 -[NSPersistentStoreCoordinator(_NSInternalMethods) executeRequest:withContext:] + 688 
10 CoreData 0x00007fff8610621b -[NSManagedObjectContext executeFetchRequest:error:] + 267 
11 FortunesHelperMVC 0x0000000100001d9d -[ImportOperation getMaxID] + 572 
12 FortunesHelperMVC 0x0000000100001f95 -[ImportOperation main] + 330 
13 Foundation 0x00007fff888f406d -[__NSOperationInternal start] + 681 
14 Foundation 0x00007fff888f3d23 ____startOperations_block_invoke_2 + 99 
15 libSystem.B.dylib 0x00007fff86a98ce8 _dispatch_call_block_and_release + 15 
16 libSystem.B.dylib 0x00007fff86a77279 _dispatch_worker_thread2 + 231 
17 libSystem.B.dylib 0x00007fff86a76bb8 _pthread_wqthread + 353 
18 libSystem.B.dylib 0x00007fff86a76a55 start_wqthread + 13 
) 
terminate called after throwing an instance of 'NSException' 

¿Alguna idea de por qué esto no está funcionando? Estoy perplejo después de mucho googlear.

Gracias

Darren.

Answer 1

El problema era que esta línea está mal:

[fetch setResultType:NSDictionaryResultType]; 

El tipo de resultado debería haber sido NSManagedObjectResultType

Saludos

Darren.

Answer 2

que tuvieron que resolver un problema similar y se acercó un poco diferente por:

• consulta de la entidad
• pedido por el atributo que desea
• limitar los resultados traiga a 1

El código para esto está a continuación. Estoy consultando una entidad llamada 'entityName' y recuperando el valor máximo para el atributo 'sequenceId'.

NSFetchRequest *request = [[NSFetchRequest alloc] init]; 
NSEntityDescription *res = [NSEntityDescription entityForName:@"entityName" inManagedObjectContext:managedObjectContext]; 
[request setEntity:res]; 

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"sequenceId" ascending:NO]; 
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil]; 
[request setSortDescriptors:sortDescriptors]; 
[sortDescriptors release]; 
[sortDescriptor release]; 

[request setFetchLimit:1]; 

NSError *error = nil; 
NSArray *results = [managedObjectContext executeFetchRequest:request error:&error]; 
[request release]; 
if (results == nil) { 
    NSLog(@"error fetching the results: %@",error); 
} 

NSInteger maximumValue = 0; 
if (results.count == 1) { 
    Result *result = (Result*)[results objectAtIndex:0]; 
    maximumValue = [result.sequenceId integerValue]; 
} 

Answer 3

Este código de Core Data Programming Guide debería funcionar.

NSFetchRequest *request = [[NSFetchRequest alloc] init]; 
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Event" inManagedObjectContext:context]; 
[request setEntity:entity]; 

// Specify that the request should return dictionaries. 
[request setResultType:NSDictionaryResultType]; 

// Create an expression for the key path. 
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"creationDate"]; 

// Create an expression to represent the maximum value at the key path 'creationDate' 
NSExpression *maxExpression = [NSExpression expressionForFunction:@"max:" arguments:[NSArray arrayWithObject:keyPathExpression]]; 

// Create an expression description using the maxExpression and returning a date. 
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init]; 

// The name is the key that will be used in the dictionary for the return value. 
[expressionDescription setName:@"maxDate"]; 
[expressionDescription setExpression:maxExpression]; 
[expressionDescription setExpressionResultType:NSDateAttributeType]; 

// Set the request's properties to fetch just the property represented by the expressions. 
[request setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]]; 

// Execute the fetch. 
NSError *error = nil; 
NSArray *objects = [managedObjectContext executeFetchRequest:request error:&error]; 
if (objects == nil) { 
    // Handle the error. 
} 
else { 
    if ([objects count] > 0) { 
     NSLog(@"Maximum date: %@", [[objects objectAtIndex:0] valueForKey:@"maxDate"]); 
    } 
}