 <p>Not sure if I understand correctly, but it sounds like you're looking for <a href="http://knockoutjs.com/documentation/computedObservables.html" rel="nofollow">Computed Observables</a>:</p> <pre><code>self.AvailableItemTypes = ko.computed(function() { var selectedTypeIds = this.SelectedItemTypes().map(function(el) { return el.Id; }); return this.ItemTypes().filter(function(el) { return selectedTypeIds.indexOf(el.Id) === -1; }); }); </code></pre>