NSPredicate返回所有对象

Question

我正在尝试使用NSPredicate来获取特定对象。问题是所有对象都被提取，它应该只是条件正确的特定对象。

- (void)fetchDevices {
    // Fetch the devices from persistent data store

    NSManagedObjectContext *managedObjectContext = [self managedObjectContext];
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Songs"];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Songs" inManagedObjectContext:managedObjectContext];
    [fetchRequest setEntity:entity];
    // retrive the objects with a given value for a certain property
    NSPredicate *predicate = [NSPredicate predicateWithFormat: @"selectedRow == %@" , selectedRowNumber];
    devices = [[managedObjectContext executeFetchRequest:fetchRequest error:nil] mutableCopy];
    [fetchRequest setPredicate:predicate];

}

selectedRowNumber是int文件中声明的.h。

看看我的歌曲实体：

PS。如果需要更多信息，请在下方发表评论，我将提供。

Answer 1

您必须在执行获取请求之前分配谓词。

另外，如果selectedRowNumber是int，那么您必须使用%d格式 而不是%@。

NSPredicate *predicate = [NSPredicate predicateWithFormat: @"selectedRow == %d" , selectedRowNumber];
[fetchRequest setPredicate:predicate];
devices = [[managedObjectContext executeFetchRequest:fetchRequest error:nil] mutableCopy];

Answer 2

无需使用setEntity设置实体。

[fetchRequest setPredicate:predicate]

只需设置谓词，就可以了。

- (void)fetchDevices {
    // Fetch the devices from persistent data store

    NSManagedObjectContext *managedObjectContext = [self managedObjectContext];
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Songs"];
    // retrive the objects with a given value for a certain property
    NSPredicate *predicate = [NSPredicate predicateWithFormat: @"selectedRow == %@" , selectedRowNumber];
    [fetchRequest setPredicate:predicate];
    devices = [[managedObjectContext executeFetchRequest:fetchRequest error:nil] mutableCopy];
    [fetchRequest setPredicate:predicate];

}

Answer 3

试试这个，

NSPredicate *predicate = [NSPredicate predicateWithFormat: @"selectedRow = %@" , [NSNumber numberWithInt: selectedRowNumber]];
[fetchRequest setPredicate:predicate];
devices = [[managedObjectContext executeFetchRequest:fetchRequest error:nil] mutableCopy];

Answer 4

似乎有两个问题。

1. 我认为您需要在[fetchRequest setPredicate:predicate];之前放置devices = [[managedObjectContext executeFetchRequest:fetchRequest error:nil] mutableCopy];

2. @"selectedRow == %d"似乎也不对。您可能需要将其更改为@"selectedRow = %d"

3. 无需再次设置实体。

所以代码可能看起来像这样。

- (void)fetchDevices {
    int selectedRowNumber = 1;
    NSManagedObjectContext *managedObjectContext = [self managedObjectContext];
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Songs"];
    NSPredicate *predicate = [NSPredicate predicateWithFormat: @"selectedRow = %d" , selectedRowNumber];
    [fetchRequest setPredicate:predicate];
    NSArray *devices = [[managedObjectContext executeFetchRequest:fetchRequest error:nil]     mutableCopy];
}

让我知道它是怎么回事:)）