<%= form_tag dispatch_call_call_path(call), :class => "dispatch-form-tag" do  %>
   <%= select_tag(:unit_ids, options_from_collection_for_select(Unit.active.order("unit_name ASC"), "id", "unit_name"), :include_blank => true, :required => true, :class => 'select' )%>
   <%= button_tag 'Dispatch', class: 'btn btn-danger btn-small', data: {disable_with: "<i class='icon-spinner'></i>Processing..."} %>
   <% end %>

def unit_select
   Unit.active.order("unit_name").map{|unit| unit.calls.where(call_status: "open").empty? ? ["#{unit.unit_name} #{unit.unit_type.unit_type}", unit.id] : ["#{unit.unit_name} (on call) #{unit.unit_type.unit_type}", unit.id] }
 end

<%= f.select(:unit_ids, unit_select,  {}, {:multiple => true, :class => 'select'}) %>