魔法记录，无法保存对象：contextDidSave == NO, error = nil

Question

我正在尝试持久化新实体：

- (void)updateStorageWithQuizzess:(NSArray *)quizzess completion:(void(^)(NSArray *quizzess, BOOL succes, NSError *error))completion {
    NSMutableArray *mutableArray = [NSMutableArray array];

    [Quiz MR_truncateAll];
    [[NSManagedObjectContext MR_context] MR_saveWithBlock:^(NSManagedObjectContext *localContext) {

        for (NSDictionary *dictionary in quizzess) {
            Quiz *quiz = [Quiz MR_createEntity];
            [quiz fromDictionary:dictionary];
            [mutableArray addObject:quiz];
        }
    } completion:^(BOOL contextDidSave, NSError *error) {
        BlockSafeRun(completion, mutableArray, contextDidSave, error);
    }];
}

或者像这样：

- (void)updateStorageWithQuizzess:(NSArray *)quizzess completion:(void(^)(NSArray *quizzess, BOOL succes, NSError *error))completion {
    NSMutableArray *mutableArray = [NSMutableArray array];

    [Quiz MR_truncateAll];

    [MagicalRecord saveWithBlock:^(NSManagedObjectContext *localContext) {
        for (NSDictionary *dictionary in quizzess) {
            Quiz *quiz = [Quiz MR_createEntity];
            [quiz fromDictionary:dictionary];
            [mutableArray addObject:quiz];
        }
    } completion:^(BOOL contextDidSave, NSError *error) {
        BlockSafeRun(completion, mutableArray, contextDidSave, error);
    }];
}

但在完成块中，我收到 contextDidSave == NO，error == nil。 所以我无法弄清楚出了什么问题。 我犯了一些明显的错误吗？我该如何调试该问题？

//////

2015-06-17 20:39:27.061 HITO[6733:618953] Set root saving context: <NSManagedObjectContext: 0x16dbe070>
2015-06-17 20:39:27.062 HITO[6733:618953] Created new main queue context: <NSManagedObjectContext: 0x16e855b0>
2015-06-17 20:39:27.063 HITO[6733:618953] Set default context: <NSManagedObjectContext: 0x16e855b0>
2015-06-17 20:39:27.316 HITO[6733:618953] [HockeySDK] WARNING: Detecting crashes is NOT enabled due to running the app with a debugger attached.
2015-06-17 20:39:28.829 HITO[6733:618953] Created new private queue context: <NSManagedObjectContext: 0x16d57870>
2015-06-17 20:39:28.831 HITO[6733:619027] Created new private queue context: <NSManagedObjectContext: 0x16ea4ec0>
2015-06-17 20:39:28.841 HITO[6733:619027] NO CHANGES IN ** saveWithBlock:completion: ** CONTEXT - NOT SAVING

更新

以下来自 MR 的代码：

- (void) MR_saveWithOptions:(MRSaveOptions)saveOptions completion:(MRSaveCompletionHandler)completion;
{
    __block BOOL hasChanges = NO;

    if ([self concurrencyType] == NSConfinementConcurrencyType)
    {
        hasChanges = [self hasChanges];
    }
    else
    {
        [self performBlockAndWait:^{
            hasChanges = [self hasChanges];
        }];
    }

    if (!hasChanges)
    {
        MRLogVerbose(@"NO CHANGES IN ** %@ ** CONTEXT - NOT SAVING", [self MR_workingName]);

        if (completion)
        {
            dispatch_async(dispatch_get_main_queue(), ^{
                completion(NO, nil);
            });
        }

        return;
    }

所以，hasChanges 返回 NO。

Answer 1

您的对象在保存块中没有发生任何更改。我在这里看到了两个问题。

1. 当您需要在 localContext（即保存块的保存上下文）中创建新对象时，您正在 MR_defaultContext 中创建新对象。

试试这个：

- (void)updateStorageWithQuizzess:(NSArray *)quizzess completion:(void(^)(NSArray *quizzess, BOOL succes, NSError *error))completion {
    NSMutableArray *mutableArray = [NSMutableArray array];


    [MagicalRecord saveWithBlock:^(NSManagedObjectContext *localContext) {
        for (NSDictionary *dictionary in quizzess) {
            Quiz *quiz = [Quiz MR_createInContext:localContext];
            [quiz fromDictionary:dictionary];
            [mutableArray addObject:quiz];
        }
    } completion:^(BOOL contextDidSave, NSError *error) {
        BlockSafeRun(completion, mutableArray, contextDidSave, error);
    }];
}

1. 在您开始导入新对象之前，您的对象实际上并未被删除。这可能不会导致您的上下文没有变化，但我也会讨论这个问题。

[Quiz MR_truncateAll] 只是将所有Quiz 对象deletedproperty 设置为true。这意味着下次保存或处理上下文时，将保存更改。

因此，当您创建新的保存上下文时，该上下文仍然具有这些对象。我不确定您的 fromDictionary 方法在做什么，但如果它依赖于数据库，那么它就不会拥有它。

你需要这样做：

- (void)updateStorageWithQuizzess:(NSArray *)quizzess completion:(void(^)(NSArray *quizzess, BOOL succes, NSError *error))completion {
    NSMutableArray *mutableArray = [NSMutableArray array];


    [MagicalRecord saveWithBlock:^(NSManagedObjectContext *localContext) {
        [Quiz MR_truncateAllInContext:localContext];
        [localContext processPendingChanges];

        for (NSDictionary *dictionary in quizzess) {
            Quiz *quiz = [Quiz MR_createInContext:localContext];
            [quiz fromDictionary:dictionary];
            [mutableArray addObject:quiz];
        }
    } completion:^(BOOL contextDidSave, NSError *error) {
        BlockSafeRun(completion, mutableArray, contextDidSave, error);
    }];
}

这样，您将删除保存上下文中的对象。您还必须记住在保存上下文中调用 processPendingChanges 以从上下文中删除对象，而不仅仅是将它们标记为要删除。