diff_sample.rb

# -*- encoding: utf-8 -*-


old_csv = <<CSV
1,aaa
2,bbb
3,ccc
4,ddd
CSV

new_csv = <<CSV
2,bbb
3,ccd
4,dde
5,eee
CSV


old_csv # => "1,aaa\n2,bbb\n3,ccc\n4,ddd\n"
new_csv # => "2,bbb\n3,ccd\n4,dde\n5,eee\n"

old_hash = {}
old_csv.each_line {|l|
  id = l.split(',')[0]
  old_hash[id] = l
}

old_hash # => {"1"=>"1,aaa\n", "2"=>"2,bbb\n", "3"=>"3,ccc\n", "4"=>"4,ddd\n"}

result = []

new_csv.each_line {|new_row|
  new_id= new_row.split(',')[0]
  old_row = old_hash[new_id]

  if new_row != old_row
    result << new_row
  end
}

result # => ["3,ccd\n", "4,dde\n", "5,eee\n"]