Question

Consider the following piece of code –

var MyObjectModel = function(myObject){
var self = this;
self.myNumber1 = ko.observable(myObject.number1).trimmed();
self.myNumber2 = ko.observable(myObject.number2).trimmed();

I would like to extend myNumber1 and myNumber2, to add knockout validations such that it throws an error if both are empty, and stays fine if either has a value.

Any idea how I could go about achieving this ?

Answer 1

You can use an ko.computed to create a function which checks if at least any of both fields is set and onlyIf in the validation to execute the validation conditionally.

var MyObjectModel = function (myObject) {
    var self = this;
    self.myNumber1 = ko.observable(myObject.number1);
    self.myNumber2 = ko.observable(myObject.number2);

    self.numbersHaveNoValue = ko.computed(function () {
        var value1 = self.myNumber1(), value2 = self.myNumber2();
        return !ko.validation.rules.required.validator(value1, true) &&
               !ko.validation.rules.required.validator(value2, true);
    });

    self.myNumber1.extend({ required: { onlyIf: self.numbersHaveNoValue }});
    self.myNumber2.extend({ required: { onlyIf: self.numbersHaveNoValue }});
}

I have created a fiddle: http://jsfiddle.net/delixfe/aBytt/