[ruby-on-rails] Rails: How does the respond_to block work?

The meta-programming behind responder registration (see Parched Squid's answer) also allows you to do nifty stuff like this:

def index
  @posts = Post.all

  respond_to do |format|
    format.html  # index.html.erb
    format.json  { render :json => @posts }
    format.csv   { render :csv => @posts }
    format.js
  end
end

The csv line will cause to_csv to be called on each post when you visit /posts.csv. This makes it easy to export data as CSV (or any other format) from your rails site.

The js line will cause a javascript file /posts.js (or /posts.js.coffee) to be rendered/executed. I've found that to be a light-weight way to create an Ajax enabled site using jQuery UI pop-ups.