保存特定對象的managedObjectContext

Question

我正在嘗試學習Core Data，並且在將新對象添加到現有對象後更新MOC時遇到問題。我可以創建原始對象，一個訓練日，並且我可以將訓練對象添加到該訓練日，但我無法弄清楚如何保存上下文，以便稍後在我的應用程序中可以在訓練日中找到所有練習。

任何想法??

這裏是我的代碼：

// Data from JSON 
NSArray *trainingDayData = responseData[@"training_days"]; 
for (NSDictionary *aTrainingDay in trainingDayData) { 

    // Find the specific training day and save the MOC, creating the trainingDayObject 
    NSNumber *idTrainingDay = [NSNumber numberWithInt:[[aTrainingDay objectForKey:kID_KEY] intValue]]; 
    VitTrainingDay *trainingDayObject = [VitTrainingDay trainingDayCreateOrObjectWithID:idTrainingDay]; 

    // Configure the VitTrainingDay object's fields 
    trainingDayObject.name = aTrainingDay[@"name"]; 
    trainingDayObject.order = aTrainingDay[@"order"];    

} 
// assign exercises to each trainingDayObject(this is inside a larger for loop) 
trainingDayObject.userExercise = [NSSet setWithArray:userExerciseObjects]; 

// Below are attempt one and two to update the MOC after assigning exercises to the trainingDayObject. 

// This works to save the updated MOC, but also adds two blank trainingDayObjects, since it 'insertNewObjectForEntityName', which I don't want. 
trainingDayObject = [NSEntityDescription insertNewObjectForEntityForName:@"TrainingDay" inManagedObjectContext:self.context]; 

//This as far as I can tell is doing nothing. It just points to the conventional MOC save method. I pull it out below. 
[self.coreDataManager saveContextForManagedObjectContext:self.context]; 

這裏是saveContextForManagedObjectContext方法我稱之爲上面：

- (void)saveContextForManagedObjectContext:(NSManagedObjectContext *)moc 
{ 
    NSError *error = nil; 
    if (![moc save:&error]) { 
     NSLog(@"Can't Save! %@ %@", error, [error localizedDescription]); 
    } 
} 

Answer 1

我有點困惑你的循環，但假設第一個是用來獲取每個培訓日，設置一些值，然後獲得當天的練習，並設置關係嘗試以下。

請注意，下面這行代碼是創建NSManagedObject的，因此您需要調用它來創建每個TrainingDay對象和每個Exercise對象。

trainingDayObject = [NSEntityDescription insertNewObjectForEntityForName:@"TrainingDay" inManagedObjectContext:self.context]; 

不知道這行做，但據推測它會創建一個新的對象，如果它已經存在

VitTrainingDay *trainingDayObject = [VitTrainingDay trainingDayCreateOrObjectWithID:idTrainingDay]; 

除非還呼籲insertNewObjectForEntityForName那麼它應該有一個調用，它被替換返回一個創建NSManagedObject或者搜索並返回一個具有匹配ID的。如果它調用insertNewObjectForEntityForName，則應刪除下面的行，因爲它只是在數據庫中創建另一個trainingDay對象而不設置任何屬性值。

嘗試這樣的事情

// Data from JSON 
    NSArray *trainingDayData = responseData[@"training_days"]; 

    for (NSDictionary *aTrainingDay in trainingDayData) { 

     // Find the specific training day ID 
     NSNumber *idTrainingDay = [NSNumber numberWithInt:[[aTrainingDay objectForKey:kID_KEY] intValue]]; 

     //Create the Core Data Object 
     //Assume VitTrainingDay is a NSManagedObject subclass 
     VitTrainingDay *trainingDayObject = [NSEntityDescription insertNewObjectForEntityForName:@"TrainingDay" inManagedObjectContext:self.context]; 

     // Set the attributes 
     trainingDayObject.ID = idTrainingDay; 
     trainingDayObject.name = aTrainingDay[@"name"]; 
     trainingDayObject.order = aTrainingDay[@"order"];    


     // assign exercises to each trainingDayObject 
     for (SomeSourceObject *object in SomeExercisesSource) { 
      ExerciseObject *exercise = [NSEntityDescription insertNewObjectForEntityForName:@"ExerciseObject" inManagedObjectContext:self.context]; 

      // Set the exercises parent object (training day) 
      exercise.trainingDay = trainingDayObject; 
      exercise.details = object.details; 

     } 

    } 


NSError *error = nil; 
if (![self.context save:&error]) { 
    NSLog(@"Can't Save! %@ %@", error, [error localizedDescription]); 
} else { 
    NSLog(@"Save successful"); 
} 

我的猜測是，你需要創建一個新的，所以如果那是什麼這個電話[VitTrainingDay trainingDayCreateOrObjectWithID:idTrainingDay];不會再使用它，而不是之前尋找具有相同ID的現有震撼教育對象。