before_filter to determine which site to serve in a multi-site Rails app

gistfile1.rb

def set_site
  if RAILS_ENV == "development"
    session[:site] = ENV['site'] || "domainA" # Default to domainA in development
  else session[:site].blank?
    if RAILS_ENV == "staging"
      session[:site] = case request.subdomains.last # *.yourstagingdomain.com
        when "domainA" then "domainA"
        when "domainB" then "domainB"
      end
    elsif RAILS_ENV == "production"
      session[:site] = case request.domain
        when "domainA.com" then "domainA"
        when "domainB.com" then "domainB"
        else "domainA"
      end
    else
      session[:site] = "domainA" # default
    end
  end
  if @site.nil?
    @site ||= Site.find_by_name(session[:site])
  end
end