admin.js

/* You must be using jQuery and jQuery UI (w/ Autocomplete)
* http://jqueryui.com/demos/autocomplete/
*/

// For Comma Splitting
function split( val ) {
	return val.split( /,\s*/ );
}
		
function extractLast( term ) {
	return split( term ).pop();
}

function autocomplete(obj, array){
	$( obj )
			// 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(
						array, 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;
				}
			});
}

users.rb

f.inputs 'Tags' do
  f.input :tag_list, :input_html => 
    { :onclick => "autocomplete($(this),#{Tag.select('name').map { |tag| tag.name}})" }