You cannot call create unless the parent is saved

models.rb

# OLD One with error => 'You cannot call create unless the parent is saved'
def number_list=(numbers_string)
    self.numbers.destroy_all
    numbers = numbers_string.split(",").collect{|num| num.strip.downcase}.uniq
    numbers.each do |number|
       num = self.numbers.find_or_create_by_content(number) 
    end
  end

remembering those days in rails.
Django Rocks!