CoreData字符串搜索

我的数据库里有个字段是name,里面的记录有一条是AA1
我现在想把他找出来，代码是这样写的+(NSManagedObject *)GetObjectFrom:(NSString *) tablename ForFieldName:(NSString *)fieldname ForValue:(NSString *) value{
    
    AppDelegate *appDelegate = (AppDelegate*)[[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *managedObjectContext = appDelegate.managedObjectContext;
    
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSEntityDescription *entityDescription = [NSEntityDescription
                                              entityForName:tablename
                                              inManagedObjectContext:managedObjectContext];
    [request setEntity:entityDescription];
    NSPredicate *pred = [NSPredicate
                         predicateWithFormat:@"(%@=%@)", fieldname,value];
    [request setPredicate:pred];
    NSError *error;
    NSArray *objects = [managedObjectContext executeFetchRequest:request
                                              error:&error];
    [request release];
    if ([objects count] > 0){
        return [objects objectAtIndex:0];
    }
    else
        return nil;
    
}我还试了
NSPredicate *pred = [NSPredicate
                         predicateWithFormat:@"(%@='%@')", fieldname,value];
或
NSPredicate *pred = [NSPredicate
                         predicateWithFormat:@"(%@==%@)", fieldname,value];
NSPredicate *pred = [NSPredicate
                         predicateWithFormat:@"(%@=='%@')", fieldname,value];好象都不行，要怎么写才可以