title: How to override constraint programmatically?
body: <p>Initially I set the height of my container as 75. But on <code>viewWillLoad</code> I want to update this value and set to <code>width / 5</code>. </p>
  
  <p>For this I try:</p>
  
  <pre><code>containerHeight = NSLayoutConstraint(item: container, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: width / 5)
  container.addConstraint(containerHeight)
  </code></pre>
  
  <p>Also:</p>
  
  <pre><code>    containerHeight.constant = width / 5
  </code></pre>
  
  <p>but both variants doesn't update my constraint. It stills returns me 75. What's wrong?</p>