str = "many words here words words words ..."
first_500_words = str.split(" ").first(500).join(" ")
first_500_chars = str[0..500]

def truncate_words(text, length = 300, end_string = ' …')
  words = text.split()
  words[0..(length-1)].join(' ') + (words.length > length ? end_string : '')
end

str = "this is really long string which I want to truncate..."

str.truncate 300, separator: " "

str.truncate(300, separator: " ")

str.split.first(300).join " "