def parse_string(string)

    # string is going to be something like:
    #   1 small cucumber, finely diced
    # or something more complex like
    #   4 x 200g tuna steaks
    if string.sub!(/^\s*(\d\S*)\s+/, '')
      self.quantity = eval($1 + ".to_f")
    end

    words = string.split(/\W+/).map {|w| w.singularize.downcase}
    if words.empty?
      errors.add( :original, "is missing or invalid")
    end

    # we want to grab each word in the words, then each pair of words, then each
    # triplet of words, etc. until all combinations have been tested against
    # existing units and foods - we exit as soon as we find a food because (in
    # English) units always preceed foods
    words.length.downto(1) do |number_of_words|
      (0..words.length - number_of_words).each do |index|
        phrase = words.slice(index, number_of_words).join(" ")
        if f = Food.find_by_name(phrase)
          self.food = f
          return true
        elsif unit.nil? and ( u = Unit.find_by_name(phrase) or ( a = Abbreviation.find_by_name(phrase) and u = a.unit ))
          self.unit = u
          words.fill(nil, index, number_of_words)
        end
      end
    end
    return !self.unit.nil?
  end