iPhone: How to copy a file from resources to documents?

I just started with iPhone development. In one of the example where I had to display some data stored in sqlite db in a table view in a tabbar controller, I had to move a sqlite file from application bundle to the documents folder.

I used the application template - iOS > Application > Window-based application for iPhone (Used core data for storage)

In the template generated by XCode (base sdk set to latest iOS = 4.2), the following code was there...

- (NSURL *)applicationDocumentsDirectory {
    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
}

In trying to get a path to documents folder, I used the method given above like this...

NSString *documentDirectory = [self applicationDocumentsDirectory];

This gave a warning - warning: incompatible Objective-C types initializing 'struct NSURL *', expected 'struct NSString *'

So I changed the code to following...

// Added the message absoluteString over here
NSString *documentDirectory = [[self applicationDocumentsDirectory] absoluteString];

NSString *writableDBPath = [documentDirectory stringByAppendingPathComponent:@"mydb.sqlite"];
NSString *defaultDBPath = [[[NSBundle mainBundle] resourcePath] stringByAppendingPathComponent:@"mydb.sqlite"];
BOOL success = [fileManager copyItemAtPath:defaultDBPath toPath:writableDBPath error:&error];
if (!success) {
   NSLog(@"Failed to create writable database file with message '%@'.", [error localizedDescription]);
}

And BOOM!!! It gave the error that - 'Failed to create writable database file with message 'The operation couldn’t be completed. No such file or directory'.'

How should I find the path to documents directory since the method applicationDocumentsDirectory generated by XCode template does not work for me.

Also, can somebo