Determining whether a user arrived from Google (and the query he used if so)

From within my Rails app, how do I determine whether a user arrived on a given page by clicking a Google (or Bing or Yahoo) search result?

If the user arrived in this fashion, ho开发者_如何学Pythonw do I determine what search terms he used?

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Have a look at

referer = @request.env["HTTP_REFERER"]

then, parse the info you're looking for (site, query) from that string.