Filter view using gem ransack with checkbox

I should have 3 checkbox OP, SE and PM. If one or two is checked or even all three, the view will filter according to the checked values. These are employee_type and if I check OP, all employees that belong to OP type will show. I have this code using gem 'ransack' on ruby on rails

On my index.html.erb

<%= search_form_for @filter do |f| %>
    <%= f.check_box :employee_type, :value => 'OP', :id => 'checkOP' %>
    <%= f.check_box :employee_type, :value => 'SE', :id => 'checkSE' %>
    <%= f.check_box :employee_type, :value => 'PM', :id => 'checkPM' %>
    <%= f.submit 'Filter' %>
<% end %>

  <script type="text/javascript">
      $('#checkOP').on('change', function() {
          var val = this.checked ? this.value : '';
          $('#index').html(val);
      });
      $('#checkSE').on('change', function() {
          var val = this.checked ? this.value : '';
          $('#index').html(val);
      });
      $('#checkPM').on('change', function() {
          var val = this.checked ? this.value : '';
          $('#index').html(val);
      });
  </script>

On my employees_controller

  def index
    @filter = Employee.search(params[:q])
    @employees = @filter.result
  end

There shouldn't be a button since it should have filtered the moment the checkbox is checked or unchecked but I use button just to test since there's something wrong with my code that won't auto-detect if the checkbox is checked or not.

There seems to have no solution in the internet or I just couldn't find the right words to search. Any help is much appreciated.