添加到关系后未在核心数据中找到对象

Question

我在添加关系后获取对象时遇到问题。我第一次获取类别时，总是找到，然后当我添加到关系时，找不到以下类别。

关系是Many-To-Many。

示例：

使用 categoryId = 10
找到类别对象
添加到父对象关系
下一个对象

如果几个分类相同id,categoryId = 10,找不到

NSManagedObjectContext *private = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];

   [private setParentContext:self.model.context];
    __block NSError *error = nil;

    [private performBlockAndWait:^{

        GPDeal *deal = [EKManagedObjectMapper objectFromExternalRepresentation:dic withMapping:[GPDeal objectMapping] inManagedObjectContext:private];
        for (NSDictionary *dic in responseObject[@"response"]) {

            GPCategory *category;

           //The first time always found
            if ((category = [GPCategory MR_findFirstByAttribute:@"catId" withValue:dic[@"mainAttribute"] inContext:private])) {
                NSLog(@"Found");
                [category addDealsObject:deal];

            } else {
                NSLog(@"Not Found");

            }

        }
    }];

    NSError *PrivateError = nil;
    if (![private save:&PrivateError]) {
        NSLog(@"Unresolved error %@, %@", PrivateError, [PrivateError userInfo]);
        abort();
    }

    if (!error) {
        //Save on main moc
        [self.model saveWithErrorBlock:^(BOOL success, NSError *error) {
            if (!success) {
                NSLog(@"Error saving context: %@\n%@", [error localizedDescription], [error userInfo]);
            }
        }];

    } else {
        NSLog(@"Error saving context: %@\n%@", [error localizedDescription], [error userInfo]);
    }

编辑：

已解决，我想我的问题是我忘记在每次迭代结束时保存主上下文。

        NSManagedObjectContext *backgroundMOC = [self.model backgroundMOC:self.model.context];

        [backgroundMOC performBlockAndWait:^{

            for (NSDictionary *dic in responseObject[@"response"]) {

                GPDeal *deal = [EKManagedObjectMapper objectFromExternalRepresentation:dic withMapping:[GPDeal objectMapping] inManagedObjectContext:backgroundMOC];

                GPCategory *category;
                if ((category = [GPCategory MR_findFirstByAttribute:@"catId" withValue:dic[@"mainAttribute"] inContext:backgroundMOC])) {
                    NSLog(@"Found with mainAttribute %@", dic[@"mainAttribute"]);
                    [deal addDealCategoryObject:category];
                }

                if([backgroundMOC hasChanges]) {
                    NSError * error;
                    [backgroundMOC save:&error];

                    [self.model.context performBlockAndWait:^{
                        if([self.model.context hasChanges]) {
                            NSError * error;
                            [self.model.context save:&error];
                        }
                    }];
                }
            }
        }];

Answer 1

您可能缺少保存MOC链。为清楚起见，我将关键字 private 替换为变量名称 backgroundMOC.

关于上述问题，我只能假设 NSManagedObjectContext *private = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; 行在技术上类似于：

- (NSManagedObjectContext *)backgroundMOC:(NSManagedObjectContext *)mainMOC
{
    NSManagedObjectContext * threadManagedObjectContext = [[NSManagedObjectContext alloc]
                             initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    [threadManagedObjectContext setParentContext:mainMOC];
    [threadManagedObjectContext setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy];
    return threadManagedObjectContext;
}

传递 self.model.context 作为 mainMOC，带有 self.model.context 和 setMergePolicy。
同样，我必须假设 self.model saveWithErrorBlock 在技术上等同于：

[mainMOC performBlockAndWait:^{
    if([mainMOC hasChanges]) {
        NSError * error;
        [mainMOC save:&error];
        // handle error
    }
}];

如果是这样，backgroundMOC（您的 private 参考）也应该这样说：

[backgroundMOC performBlockAndWait:^{
    if([backgroundMOC hasChanges]) {
        NSError * error;
        [backgroundMOC save:&error];
        // handle error
    }
}];

换句话说，您要确保 backgroundMOC 和 mainMOC 保存操作是从各自的线程执行的，performBlockAndWait.

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