How to check if a featured news item already exists in a Rails Department

I'm creating a news app in Rails and it has a carousel that shows the news stories highlighted. Each department has a place to show this prominence. I would like to know how to make an alert appear as soon as the "Featured News" checkbox is selected, asking if the user wants to change the highlighted news in a department (in case there is already a featured news in the respective department).

_form.html

    <div id="highlight" class="field">
        <%= f.label "Notícia em Destaque" %>
        <%= f.check_box :highlight, :"data-selects-visibility" => ".triggered_by_checkbox" %>
     </div>
<div class="field">
    <%= f.label :department_id %><br />
    <%= f.collection_select(:department_id , Department.all, :id, :name, options ={:prompt 

=> "Select department"}) %>
  </div>

_carousel.html

<div id="mycarousel" class="carousel slide" data-ride="carousel">
  <!--Indicators -->
  <ol class="carousel-indicators">
    <li data-target="#mycarousel" data-slide-to="0" class="indicator active"></li>
    <li data-target="#mycarousel" data-slide-to="1" class="indicator"></li>
    <li data-target="#mycarousel" data-slide-to="2" class="indicator"></li>
    <li data-target="#mycarousel" data-slide-to="3" class="indicator"></li>
    <li data-target="#mycarousel" data-slide-to="4" class="indicator"></li>
    <li data-target="#mycarousel" data-slide-to="5" class="indicator"></li>
  </ol>

  <div class="carousel-inner" role="listbox">
    <% @carousel_content.each do |publication| %>
      <div class="item">
        <%= link_to publication do%>
        <img src="<%= publication.image %>">
        <div class="carousel-caption">
          <h3><%= publication.image_description %></h3>
        </div>
        <% end %>
      </div>
    <% end %>
  </div>