# given a start word of foobar check caches for ["fooba", "foob", "foo", "fo", "f"]
# to see if any of the simpler queries already had a complete response
def cached_autocomplete(start)
  limit = 1000
  short = start.dup
  shorter = Array.new(start.size - 1){ short.chop!.dup }

  cached = Rails.cache.read_multi(*shorter.map { |s| "autocomplete-#{s}" })
  cached.each_value do |tags|
    if tags.size < limit # complete response / nothing is missing
      tags.select! { |t| t.start_with?(start) } # filter non-matches
      return tags
    end
  end

  Rails.cache.fetch("autocomplete-#{start}", expires_in: 15.minutes, race_condition_ttl: 1.minute) do
    ... hit the db ... limit(limit) ...
    ... prevent multiple cache refreshes with race_condition_ttl ...
  end
end