_autocomplete.js.erb

<script>
	$(document).ready(function(){
	  $(function() {
	    var availableTags = <%=raw @all_tags %>;
		
	  		function split( val ) {
	  			return val.split( /,\s*/ );
	  		}
	  		function extractLast( term ) {
	  			return split( term ).pop();
	  		}

	  		$( "#blog_post_tag_list, #news_item_tag_list, #portfolio_entry_tag_list" )
	  			// don't navigate away from the field on tab when selecting an item
	  			.bind( "keydown", function( event ) {
	  				if ( event.keyCode === $.ui.keyCode.TAB &&
	  						$( this ).data( "autocomplete" ).menu.active ) {
	  					event.preventDefault();
	  				}
	  			})
	  			.autocomplete({
	  				minLength: 0,
	  				source: function( request, response ) {
	  					// delegate back to autocomplete, but extract the last term
	  					response( $.ui.autocomplete.filter(
	  						availableTags, extractLast( request.term ) ) );
	  				},
	  				focus: function() {
	  					// prevent value inserted on focus
	  					return false;
	  				},
	  				select: function( event, ui ) {
	  					var terms = split( this.value );
	  					// remove the current input
	  					terms.pop();
	  					// add the selected item
	  					terms.push( ui.item.value );
	  					// add placeholder to get the comma-and-space at the end
	  					terms.push( "" );
	  					this.value = terms.join( ", " );
	  					return false;
	  				}
	  			});
	  	});
	})
</script>

controller.rb

before_filter :find_all_tags

def find_all_tags
    @all_tags = BlogPost.tag_counts_on(:tags).collect { |t| t.name }.to_a
    @all_tags = @all_tags.blank? ? '[]' : @all_tags
  end

# i understand that this code might be a bit misguided and redundant.