如何在Core Data中选择特定数据？

Question

我有一个使用Core Data并在其中加载sqlite数据库的示例程序.我正在使用此代码显示所有值

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Name"
                                          inManagedObjectContext:context];
[fetchRequest setEntity:entity];
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];

for (Name *name in fetchedObjects) {
    NSLog(@"ID: %@", name.nameId);
    NSLog(@"First Name: %@", name.first);
    NSLog(@"Middle Name: %@", name.middle);
    NSLog(@"Last Name: %@", name.last);
}

这段代码工作正常,我的问题是我无法选择特定的数据/记录.示例我想选择ID = 1的记录.

谢谢!

Answer 1

您必须在请求执行之前使用谓词(使用NSPredicate).

像这样的东西:

NSPredicate *predicateID = [NSPredicate predicateWithFormat:@"nameID == %d",-1];
[fetchRequest setPredicate:predicateID];

Answer 2

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Name"
                                          inManagedObjectContext:context];
[fetchRequest setEntity:entity];

//Add following method to your code. this will help you to get desired result.
[fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"yourColumnID == %@", yourID]];

NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];

for (Name *name in fetchedObjects) {
    NSLog(@"ID: %@", name.nameId);
    NSLog(@"First Name: %@", name.first);
    NSLog(@"Middle Name: %@", name.middle);
    NSLog(@"Last Name: %@", name.last);
}