Set validation rule depending on another rule?

[eluser]K-Fella[/eluser]
I have the following form:

Text Field: Reg Number
Text Field: Odometer Reading - optional
Radio Group: Odometer Type - optional
- Radio 1: Miles
- Radio 2: Kilometers

Notice how both Odometer Reading and Odometer Type are optional to begin with. What I'd like to do is change the Odometer Type Radio Group to required ONLY if Odometer Reading has been entered.

Is this possible?

[eluser]zdknudsen[/eluser]
I think you'll have to use this:

Code:

if ($this->post->input('odometer_reading')) {
    $rules['odometer_type'] = "required";
}