Knockout Validation - have input validate some other field

Question

I'm using knockout validation and trying to accomplish what should be a simple use case.

I want an input to be bound to a certain field, but validate a different one. Something like this:

<input type="text" data-bind="validationElement: referrerId, value: referrerName" />  

But it seems to always (correctly) show validation errors for referrerName instead of referrerId. Is there any way to get this to work?

Basically I want this fiddle:

http://jsfiddle.net/W3pQt/1/

to show an error for referrerId (which in the fiddle is always empty).

Answer 1

http://jsfiddle.net/W3pQt/3/

I made a custom validator, because I couldn't figure out how to do it without one.

ko.validation.rules['dependsOn'] = {
    validator: function (val, otherVal) {
        return typeof otherVal() !== "undefined";
    },
    message: 'Referrer Id must have a value: {0}'
};

ko.validation.registerExtenders();

...

this.referrerName.extend({ dependsOn: this.referrerId });

Don't get hung up on the name 'dependsOn' which is dumb. Naming things is hard.