Is there a way to extend validation on Knockout Validation's validatedObservable?
<p>I need to make sure each observable is valid and also make sure all the observables are valid together. In my simplified example I have a modelNumber and stockNumber that are each required, but together they must also form a valid partNumber. My attempt gives me this error:</p>

<p>"Uncaught Error: Cannot write a value to a ko.computed unless you specify a 'write' option. If you wish to read the current value, don't pass any parameters."</p>

<p>What's the problem?</p>

<p><a href="https://jsfiddle.net/yekr2ov2/18/" rel="nofollow">https://jsfiddle.net/yekr2ov2/18/</a></p>

<pre><code>// enable validation
ko.validation.init();

ko.validation.rules['partNumberValid'] = {
  validator: function(partInfo, otherVal) {
    return partInfo.modelNumber() + partInfo.stockNumber() == "m1s1";
  },
  message: 'part number is invalid'
};
ko.validation.registerExtenders();

function VM() {
  var self = this;

  self.modelNumber = ko.observable().extend({ required: true });
  self.stockNumber = ko.observable().extend({ required: true });
  self.vinNumber   = ko.observable().extend({ required: true });

  self.isPartNumberValid = ko.validatedObservable({
    modelNumber: self.modelNumber,
    stockNumber: self.stockNumber,
    vinNumber: self.vinNumber
  }).extend({ partNumberValid: true });

  self.message = ko.observable();

  self.save = function() {
    if (self.isPartNumberValid.isValid()) {
      self.message("saved");
    } 
    else {
      debugger;
      //self.isPartNumberValid.errors.showAllMessages();
      self.message("not saved");
    }    
  };

}

ko.applyBindings(new VM());
</code></pre>