example how restore session in Rails 3

gistfile1.rb

## in controller
# skip verify_authenticity_token when data from another server
skip_before_filter :verify_authenticity_token, :only => [:restore]

def restore
  restore_session_from(params[:session_id])
  redirect_to '/'
end

## ApplicationController
def restore_session_from(session_id)
  restored_session = Session.find_by_session_id(session_id)
  if restored_session
    session.update(restored_session.data)
    restored_session.destroy
  else
    Rails.logger.error("Can't find session_id #{session_id} for restore session.")
  end
end