(function($) {
      $.fn.prepopulate = function(dependencies, maxLength) {
          /*
              Depends on urlify.js
              Populates a selected field with the values of the dependent fields,
              URLifies and shortens the string.
              dependencies - array of dependent fields ids
              maxLength - maximum length of the URLify'd string
          */
          return this.each(function() {
              var prepopulatedField = $(this);
  
              var populate = function () {
                  // Bail if the field's value has been changed by the user
                  if (prepopulatedField.data('_changed')) {
                      return;
                  }
  
                  var values = [];
                  $.each(dependencies, function(i, field) {
                      field = $(field);
                      if (field.val().length > 0) {
                          values.push(field.val());
                      }
                  });
                  prepopulatedField.val(URLify(values.join(' '), maxLength));
              };
  
              prepopulatedField.data('_changed', false);
              prepopulatedField.change(function() {
                  prepopulatedField.data('_changed', true);
              });
  
              if (!prepopulatedField.val()) {
                  $(dependencies.join(',')).keyup(populate).change(populate).focus(populate);
              }
          });
      };
  })(django.jQuery);