A digital root is the recursive sum of all the digits in a number. Given n, take the sum of the digits of n. If that value has more than one digit, continue reducing in this way until a single-digit number is produced. This is only applicable to the natural numbers.

sum_of_digits_digital_root.rb

def digital_root(n)
  return n if n < 10

  until n.to_s.size == 1
   n = n.to_s.chars.map(&:to_i).reduce(:+)
  end
  
  return n
end

###### Better answer to reminber 

def digital_root(n)
  n < 10 ? n : digital_root(n.digits.sum)
end