Validation

[eluser]xwero[/eluser]
I don't know if it is going to work but you could extend the validation library with a user_exists rule

Code:

MY_Validation extends CI_Validation
{
    function user_exists($user,$password)
    {
       // database code
       return (exists)?TRUE:FALSE;
    }
}

in the controller you would have to do something like

Code:

$rules['username'] = "user_exists[password]";

Then you have to add the user_exists message to the validation language file.
Again i don't know if it works but if it does it seems to be a better way to extend the rules than using callbacks. The only drawback is the compatibility with future versions of the CI validation class.