order sample?

gistfile1.rb

# "created_at/desc" とか書けるようにしていいなら(delimiter は相談.)

order_strings.map{|o|

  name, order = o.split('/', 2)
  column_names, 
  order = 'asc' if order != 'desc'

  column_names.include?(name) ? "#{name} #{order}" : nil

}.compact.join(', ')

# あるいは

order_strings.map{|o|

  _, name, order = o.match(/\A(.+?)(?:_(desc|asc))?\z/).to_a
  next unless column_names.include?(name)
  "#{name} #{order}"

}.compact.join(', ')

# あとは map + join じゃなく each_with_object を使うとか……