只有一个值被写入核心数据。为什么？答案

【问题标题】：Only one value is being written into Core Data. Why ?只有一个值被写入核心数据。为什么？
【发布时间】：2012-01-20 12:48:21
【问题描述】：

我是新手，我想从Core Data 数据库中存储和检索数据。我从一个与 SQL 数据库通信的 php 文件中获取所有数据。这个 php 文件返回一个 JSON 对象，然后由我的应用程序解析，然后通过执行以下操作写入Core Data：

AppDelegate.m

-(void)writeDataIntoCoreData
{
    NSManagedObjectContext *context = [self managedObjectContext];
    NSManagedObject *articles = [NSEntityDescription
                                 insertNewObjectForEntityForName:@"Articles" 
                                 inManagedObjectContext:context];

    for(int i= 0; i<[json count];i++){
        NSDictionary *info = [json objectAtIndex:i];

        NSNumberFormatter * f = [[NSNumberFormatter alloc] init];
        [f setNumberStyle:NSNumberFormatterDecimalStyle];
        NSNumber * myNumber = [f numberFromString:[info objectForKey:@"userID"]];

        [articles setValue:myNumber forKey:@"articleID"];
        [articles setValue:[info objectForKey:@"username"] forKey:@"author"];
        [articles setValue:[info objectForKey:@"user_pic"] forKey:@"text"];
        NSLog(@"Name = %@",[info objectForKey:@"username"]);
        NSLog(@"Text = %@",[info objectForKey:@"user_pic"]);
    }
    NSError *error;
    if (![context save:&error]) {
        NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
    }


}

MainViewController.m

这里我尝试通过遍历fetchedObjects 来检索Core Data 数据库中的所有数据。

-(IBAction)showCD:(id)sender
{

    NSManagedObjectContext *context = [self managedObjectContext];
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription 
                                   entityForName:@"Articles" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];
    NSError *error;
    NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
    for (NSManagedObject *info in fetchedObjects) {
        NSLog(@"id: %@", [info valueForKey:@"articleID"]); 
        NSLog(@"Name: %@", [info valueForKey:@"author"]);
        NSLog(@"Text: %@", [info valueForKey:@"text"]);


    }    

}

然而，NSLog 只显示一个对象，即第一个写入数据库的对象。怎么了？

【问题讨论】：

标签： iphone objective-c xcode json core-data

【解决方案1】：

移动你的

NSManagedObject *articles = [NSEntityDescription
                             insertNewObjectForEntityForName:@"Articles" 
                             inManagedObjectContext:context];

进入你的 for 循环，

for(int i= 0; i<[json count];i++){
    NSManagedObject *articles = [NSEntityDescription
                             insertNewObjectForEntityForName:@"Articles" 
                             inManagedObjectContext:context];
    NSDictionary *info = [json objectAtIndex:i];

    NSNumberFormatter * f = [[NSNumberFormatter alloc] init];
    [f setNumberStyle:NSNumberFormatterDecimalStyle];
    NSNumber * myNumber = [f numberFromString:[info objectForKey:@"userID"]];

    [articles setValue:myNumber forKey:@"articleID"];
    [articles setValue:[info objectForKey:@"username"] forKey:@"author"];
    [articles setValue:[info objectForKey:@"user_pic"] forKey:@"text"];
    NSLog(@"Name = %@",[info objectForKey:@"username"]);
    NSLog(@"Text = %@",[info objectForKey:@"user_pic"]);
}

【讨论】：

啊是啊！非常感谢你！ :)

【解决方案2】：

将调用移至循环中的insertNewObjectForEntityForName:inManagedObjectContext:inside。您现在的做法是，您只创建一个托管对象，然后反复更改其属性。

【讨论】：