$(document).ready(function(){
  
     $('#users_country_id').change(function(){

       $('#users_city_id option').remove();

       value = $(this).val();
       
       if(value == '') return false;
       
       $.getJSON("/ajax/get_city/"+value, function(data){
        
          $.each(data, function(key, val){
            $('#users_city_id').append('<option value='+val.id+'>'+val.title+'</option>');
          });
          
        });

     })
  });