title: Shiny Update Input Selector issues
body: <p>I am trying to use the updateSelectInut in Shiny Dashboard to no avail. I have tried multiple methods but am not able to get the selectInput's to update. Data has a VG and AG columns with several factors each. What I want to do is when I select a VG, then this updates with only the AG values that exist for that VG file:</p>
  
  <pre><code>VG  AG
  A    1
  A    2
  B    1
  B    3
  </code></pre>
  
  <p>Such that if VG is selected then the AG options will be 1,3.</p>
  
  <p>Code is as follows:</p>
  
  <pre><code>UI&lt;-...
  
  column(3,selectInput("VG.ETD", label = h4("VG"),choices = levels(data$VG),selected ="Y1")),
  column(3,selectInput("AC.ETD", label = h4("AC"),choices = levels(data$AC), selected ="X1")),
  
  Server&lt;-function(input, output,session){...
  
  df_b &lt;- reactive({filter(data, VG== input$VG.ETD)})
  observe({updateSelectInput(session, 'AC.ETD', choices=levels(droplevels(df_b()$AC)))})
  </code></pre>