如何使用謂詞從 Coredata 中獲取單個對象

Question

我試圖從我的核心數據庫中獲取單個對象，但它一直返回 null。 我的方法基於另一種方法，該方法返回我正在訪問的 coredata 對象中的每個值。

我以前從未嘗試過，也曾嘗試閱讀蘋果文檔，但它只是沒有意義……這就是我的方法的樣子

- (NSMutableArray *)readSelectedInstall:(NSString *)projIDString {
    NSManagedObjectContext *context = [self managedObjectContext];

    if (context == nil) {
        NSLog(@"Nil");
    }
    else {


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

        NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ProjID==%@", projIDString];
        [fetchRequest setPredicate:predicate];

        NSError *error;

        NSMutableArray *installProjectDictionaryArray = [[NSMutableArray alloc] init];


        NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
        for (InstallProject *installProj in fetchedObjects) {

            NSMutableDictionary *tempInstallProjectDictionaryArray = [[ NSMutableDictionary alloc] init];

            [tempInstallProjectDictionaryArray setObject:installProj.companyName forKey:@"CompanyName"];
            [tempInstallProjectDictionaryArray setObject:installProj.projNo forKey:@"ProjNo"];
            [tempInstallProjectDictionaryArray setObject:installProj.projID forKey:@"ProjID"];

            [installProjectDictionaryArray addObject:tempInstallProjectDictionaryArray];
        }

        return installProjectDictionaryArray;
    }
    return nil;
}

任何讓我返回 projID 與 projIDString 匹配的單個項目的幫助將不勝感激。

Answer 1

導入 NSManagedObject(InstallProject) 並像這樣獲取一個對象，

-(InstallProject *)readSelectedInstall:(NSString *)projIDString
{

    NSArray *fetchedObjects;
    NSManagedObjectContext *context = [self managedObjectContext];
    NSFetchRequest *fetch = [[NSFetchRequest alloc] init];
    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"InstallProject"  inManagedObjectContext: context];
    [fetch setEntity:entityDescription];
    [fetch setPredicate:[NSPredicate predicateWithFormat:@"(ANY ProjID contains[cd] %@)",projIDString]];
    NSError * error = nil;
    fetchedObjects = [context executeFetchRequest:fetch error:&error];

    if([fetchedObjects count] == 1)
    return [fetchedObjects objectAtIndex:0];
    else
    return nil;  

}

Answer 2

fetchLimit上的fetchRequest你可以把它設置為1

獲取限制指定請求在執行時應返回的最大對象數。

例如在斯威夫特：

let fetchRequest = NSFetchRequest(entityName: "Cart")
fetchRequest.fetchLimit = 1
fetchRequest.sortDescriptors = [NSSortDescriptor(key: #keyPath(Cart.creationDate), ascending: true)]

編輯： sortDescriptors的設置只是因為我將這個fetchRequest與 NFRC 一起使用，您也可以執行fetchRequest ：

do {
  let fetchRequest = NSFetchRequest(entityName: "Cart")
  fetchRequest.fetchLimit = 1
  fetchRequest.predicate = NSPredicate(format: "name == %@", mainCart)
  var objects: [Cart]
  try objects = mainMoc.execute(fetchRequest) as! [Cart]
} catch {
  
}