constants and custom config settings

[eluser]rizkhan[/eluser]
[quote author="InsiteFX" date="1302181063"]1) place your config file in application/config/filename.php
2) for constants you can add them to application/config/constants.php

The constants file will be autoloaded by CodeIgniter.

To load your config file to as toopay has stated above.

InsiteFX[/quote]

I have created a database_tables.php in config folder which contains constants for all database table names and loaded this file in controller class

$this->config->load('database_tables');

but when i try to access in model class for my db query table name then it shows me error

An Error Was Encountered

Your application/config/database_tables.php file does not appear to contain a valid configuration array.

for constant do we need to declare an array for constant?

thanks for your help