<% @y = [] %>
  <% User.all.each do |user| %>
  <% x = user.username %><br>
  <% if x %>
  <% @y << "@" + x %>
  <% end %>
  <% end %>
  <% @y %>

<script>
  $( function() {
    var availableTags = <%= raw @y %>;
    var inputTag = document.getElementById('tags');
    var textValue = inputTag.value;
    var at = document.getElementById("tags").value.indexOf("@");
    if (at == 1) {
          $( "#tags" ).autocomplete({
            source: availableTags
          });
        }
  } );
  </script>