取回核心數據的重複記錄

Question

夥計，

我在我的iPhone項目中有一個涉及Core Data的益智遊戲。在兩個獨立的視圖控制器實例viewDidLoad方法我調用下面的方法：

- (NSFetchedResultsController *)getStudentResults:(NSString *)classRoom forWeekStarting:(NSDate *)startDate andEnding:(NSDate *)endDate { 
    AttendanceAppDelegate *appDelegate = (AttendanceAppDelegate *)[[UIApplication sharedApplication] delegate]; 

    NSFetchRequest *request = [[NSFetchRequest alloc] init]; 
    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"Student" inManagedObjectContext:[appDelegate managedObjectContext]]; 

    int secondsInAWeek = 60 * 60 * 24 * 7; 
    NSDate *today = [[NSDate alloc] init]; 
    NSDate *nextWeek = [[NSDate alloc] initWithTimeIntervalSinceNow:secondsInAWeek]; 
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(student.classRoom like %@) AND (dateTime >= %@) AND (dateTime <= %@)", classRoom, startDate, endDate]; 
    [request setPredicate:predicate]; 
    [request setEntity:entityDescription]; 

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"dateTime" ascending:YES]; 
    NSArray *descriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil]; 
    [request setSortDescriptors:descriptors]; 
    [descriptors release]; 
    [sortDescriptor release]; 
    [nextWeek release]; 
    [today release]; 


    NSFetchedResultsController *fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:[appDelegate managedObjectContext] sectionNameKeyPath:nil cacheName:@""]; 
    NSError *error; 
    if (![fetchedResultsController performFetch:&error]) 
     NSLog(@"Error performing fetch on fetchedResultsController: %@", [error localizedDescription]); 

    if (fetchedResultsController == nil || [[fetchedResultsController fetchedObjects] count] == 0) { 
     Student *student = [NSEntityDescription insertNewObjectForEntityForName:@"Student" inManagedObjectContext:[appDelegate managedObjectContext]]; 
     NSData *classRoomStudentData = [[NSData alloc] initWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"STUDENT_ATTENDANCE" ofType:@"XML"]]; 
     [student setClassRoom:@"default"]; 
     [student buildWithStudentData:classRoomStudentData startingWithXPathNode:@"//attendance"]; 

     NSError *error; 
     if (![[appDelegate managedObjectContext] save:&error]) { 
      NSLog(@"Error saving the managedObjectContext: %@", [error localizedDescription]); 
     } 

     if (![fetchedResultsController performFetch:&error]) 
      NSLog(@"Error performing fetch on fetchedResultsController: %@", [error localizedDescription]); 
    } 
    return fetchedResultsController; 
} 

每次我跑我的應用程序的學生對象的數量由原來的實體的數量增加（每個原始實體重複一次）。所以，如果我從4開始，下一次我得到8，然後12，然後16等。我似乎無法弄清楚爲什麼，因爲解析XML的代碼僅在fetch返回對象時才被調用。

任何幫助，將不勝感激，謝謝。

邁克爾 -

Answer 1

您插入不只是當抓取根本沒有返回對象的新的對象：看你的代碼

if (fetchedResultsController == nil || [[fetchedResultsController fetchedObjects] count] == 0) 

還插入他們當你不能正確初始化NSfetchedResultsController。因此，如果您反覆無法正確初始化，每次添加新對象時都會失敗。

一個建議是初始化NSfetchedResultsController，如Apple提供的所有示例代碼所示。基本上，您首先驗證fetchedResultsController是否爲零。如果不是，你只需返回變量。否則，您將其初始化。

見我回答這個問題的一個例子：

How to properly configure NSFetchedResultsController