问无法在iOS中的核心数据中替换一个实体

EN

    NSManagedObjectContext *context = [[MyDB sharedInstance] managedObjectContext];

    User *user = [NSEntityDescription insertNewObjectForEntityForName:@"User" inManagedObjectContext:context];

    NSNumber *userNumber = 12345;

    user.id = userNumber;
    user.name = @"John Doe";
    user.email = @"john@doe.net";
    user.createdDate = [NSDate date];

    [[MyDB sharedInstance] updateUser:user];

-(void)updateUser:(User *)user {

    NSError *error;
    NSManagedObjectContext *context = [[MyDB sharedInstance] managedObjectContext];

    // Create fetch request
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"User" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];

    // Create predicate
    NSPredicate *pred = [NSPredicate predicateWithFormat:@"id == %@", user.id];
    [fetchRequest setPredicate:pred];

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

    if (error) {
        // handle fetch error
    } else {

        user = [[User alloc] initWithEntity:entity insertIntoManagedObjectContext:context];

        for (User *recordToDelete in results) {

            [context deleteObject:recordToDelete];//record gets deleted here, which is fine

        }

        [context save:&error]; //this doesn't save the new entity that I passed in

        if (error) {
            // handle save error
            NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
        }

    }


    [fetchRequest setEntity:entity];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"id=%@", 12345];
    [fetchRequest setPredicate:pred];

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

    for (User *testObject in items) {

        NSLog(@"ID: %@, Name: %@, Email: %@, Created Date: %@", [testObject id], [testObject name], [testObject email], [testObject createdDate]);

    }

}