using values from config in the db config file

[eluser]cwpollock[/eluser]
I'm trying to centralize the change in configuration information into one file.

I want to set the db configuration along this lines

Code:

$db['default']['database'] = $this->config->item("db_name");

When I tried this it tells me that "this" is not available in this context.

Any thoughts on how to do this?