we need to specify three things for the input field: it must have `autocomplete` as class attribute so that jQuery will be able to attach events to it, and specify the ruby class and method (klass, method) we want to invoke. It's also a good idea to disable the browser autocomplete feature that would interfere with our autocomplete. Specifying klass and method may seem redundant as they could appear in the ID, but it's not: player_picture_count is an example of ambiguous ID. 

the autocomplete javascript attaches and handles keyboard and mouse events for all the input fields of `autocomplete` class and the associated autocomplete list div. In our example, the player_username field will generate requests to /autocomplete/player/username/query when the length of the input is at least 2 and the ASCII code for the pressed key is between 45 and 106 (letters, numbers and some symbols), or a backspace. Arrows, clicks and tabs are used to interact with the autocomplete list.

the middleware responds to autocomplete requests with some magic*. It's important to whitelist every single class and method that are allowed to avoid issues: the ensure_autocompletable will halt the execution otherwise. There are two possibliy configurable elements in the query: the search by wildcard - that is usually a use case expected by the user - and a limit of 10 items to return. Also, generating HTML here... TODO: refactor.

the js partial rendered by the autocomplete simply populates the autocomplete div with the result from the query and attaches some event handling for highlighting the currently selected item in the autocomplete list.