anchor.diff:  
           <div class="block donor">
             <h3>Donor</h3>
  -          <select name="donors_ids" multiple placeholder="All donors">
  +          <select name="donors_ids[]" multiple placeholder="All donors">
               <% @donors.each do |donor| %>
                 <option value="<%= donor.id %>"><%= donor.title %></option>
               <% end %>
  
positive.code: 
          <div class="block organization">
            <h3>Organization</h3>
            <select name="organizations_ids" multiple placeholder="All organizations">
              <% @organizations.each do |organization| %>
                <option value="<%= organization.id %>"><%= organization.title %></option>
              <% end %>
  
negative.code:         <% else %>
            <div class="block sectors">
              <h3><%= @site.word_for_clusters %></h3>
              <select name="sectors_ids[]" multiple placeholder="All sectors">
                <% @sectors.each do |sector| %>
                  <option value="<%= sector.id %>"><%= sector.title %></option>
                <% end %>
  