title: Update selectInput on change of another selectInput
body: <p>I need to change the value of one <code>selectInput</code>, with its own input id, whenever the value of another <code>selectInput</code>, with a different input id changes.</p>
  
  <p>In javascript this would be <code>onchage</code>, but I can't figure out how to interface the two.</p>
  
  <p><em>jQuery</em>, or <em>R</em> code is OK. </p>
  
  <p><em>EDIT:</em> Got it to work with this code: </p>
  
  <pre><code>observe({ 
      choices &lt;- input$product
      updateSelectInput(session, "productexperiance", 
                          choices=c("NA", paste(ProductList())), selected="NA")
    })
  </code></pre>
  
  <p>Still I don't understand why it works. </p>