Store formatted duration like 03:30 in seconds

song.rb

class Song < ActiveRecord::Base
  def length=(length)
    return unless length =~ /\A\d+:\d{2}\z/
    minutes, seconds = length.split(':').map(&:to_i)
    write_attribute(:length, minutes * 60 + seconds)
  end
  
  def length
    return unless length = read_attribute(:length)
    "%02d:%02d" % length.divmod(60)
  end
end

song_test.rb

require 'test_helper'

class SongTest < ActiveSupport::TestCase
  setup do
    @song = Song.new(length: '01:30')
  end
  
  test "length=(length)" do
    assert_equal 90, @song.read_attribute(:length)
  end
  
  test "length" do
    assert_equal '01:30', @song.length
  end
end