title: Validate model using KnockoutJS
body: <p>I have a form (new poll form) in which I can add multiple items (poll questions) to a collection using KnockoutJS. I want to know how can I force the user to enter a value into the question textbox before he can add a new row...</p>
  
  <p><img src="https://i.stack.imgur.com/YkNXP.png" alt="enter image description here"></p>
  
  <pre><code>var App = function (pollData, pollQuestionData) {
               var self = this;
               self.poll = new Poll(pollData);
               self.questions = ko.observableArray([new PollQuestion(pollQuestionData)]); // Put one question in by defaul
  
               // Add new question function... here should go validation I guess???
  
               self.addQuestion = function () {
                   self.questions.push(new PollQuestion(pollQuestionData))
               };
  
               // Remove question function
               self.removeQuestion = function (question) {
                   self.questions.remove(question)
               };
           };
  </code></pre>
  
  <p>Here is my jsfiddle code (that by the way I couldn't make it run...)</p>
  
  <p><a href="http://jsfiddle.net/AcE2V/3/" rel="nofollow noreferrer">http://jsfiddle.net/AcE2V/3/</a></p>