计算核心数据属性的最大值 - NSCFNumber 错误

Question

我需要找出核心数据实体的一个属性的最大值。

我仍处于 Cocoa 学习曲线中，这是我用来学习的一个简单的测试应用程序。

该应用程序从文本文件中导入财富并在屏幕上显示一个表格。导入在单独的后台线程中完成。

我在网上找到了这段代码，我试图让它工作：

- (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);  
} 

我的实体名为“Fortune”，属性名为“id”（一个 Double）。

当代码运行时，它会在执行获取请求时崩溃。控制台显示：

2009-12-18 00:53:42.777 FortunesHelperMVC[4027:1703]-[NSCFNumber count]：无法识别的选择器发送到实例 0x1004d7b10 2009-12-18 00:53:42.778 FortunesHelperMVC [4027:1703] 引发了未捕获的异常 2009-12-18 00:53:42.778 FortunesHelperMVC[4027:1703]-[NSCFNumber count]：无法识别的选择器发送到实例 0x1004d7b10 2009-12-18 00:53:42.779 FortunesHelperMVC[4027:1703] *** 由于未捕获的异常“NSInvalidArgumentException”而终止应用程序，原因：“-[NSCFNumber 计数]：无法识别的选择器发送到实例 0x1004d7b10” *** 第一次抛出调用堆栈： ( 0 核心基础 0x00007fff83ed9444 __exceptionPreprocess + 180 1 libobjc.A.dylib 0x00007fff85fbb0f3 objc_exception_throw + 45 2核心基础0x00007fff83f321c0 + [NSObject（NSObject）不识别选择器：] + 0 3 核心基础 0x00007fff83eac08f ___转发___ + 751 4 核心基础 0x00007fff83ea81d8 _CF_forwarding_prep_0 + 232 5 基础 0x00007fff88a5609e +[_NSPredicateUtilities max:] + 46 6 基础 0x00007fff8893ce72 -[NSFunctionExpression 表达式ValueWithObject:context:] + 530 7 核心数据 0x00007fff8613b5b1 -[NSMappedObjectStore executeFetchRequest:withContext:] + 2081 8 核心数据 0x00007fff8613ad10 -[NSMappedObjectStore executeRequest:withContext:] + 80 9 核心数据 0x00007fff86108900 -[NSPersistentStoreCoordinator(_NSInternalMethods) 执行请求：withContext:] + 688 10 核心数据 0x00007fff8610621b -[NSManagedObjectContext executeFetchRequest:error:] + 267 11 FortunesHelperMVC 0x0000000100001d9d -[ImportOperation getMaxID] + 572 12 FortunesHelperMVC 0x0000000100001f95 -[ImportOperation main] + 330 13 基础 0x00007fff888f406d -[__NSOperationInternal start] + 681 14 基础 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 ) 在抛出“NSException”实例后调用终止

任何想法为什么这不起作用？在谷歌上搜索了很多之后，我被难住了。

谢谢

达伦。

Answer 1

问题是这一行是错误的：

[fetch setResultType:NSDictionaryResultType];  

结果类型应该是 NSManagedObjectResultType

问候

达伦。

Answer 2

Core Data Programming Guide 的这段代码应该可以工作。

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"]);
    }
}

Answer 3

我必须解决一个类似的问题，但处理方式略有不同：

• 查询实体
• 按您想要的属性排序
• 将提取结果限制为 1

代码如下。我正在查询一个名为“entityName”的实体并检索属性“sequenceId”的最大值。

    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];
}