compat.rb

# the old way was md5

class Foo
  def digest
    Digest::MD5.hexdigest(read_value)
  end
end

# the new way is sha1

class Foo
  def digest
    Digest::SHA1.hexdigest(read_value)
  end
end

# you need to support both

class Foo
  def digest
    if old?
      Digest::MD5.hexdigest(read_value)
    else
      Digest::SHA1.hexdigest(read_value)
    end
  end
end

# but you don't want these types of conditions scattered around your code

# you could do this:

class Foo
  def digest
    Digest::SHA1.hexdigest(read_value)
  end
end

class FooCompat < Foo
  def digest
    if old?
      Digest::MD5.hexdigest(read_value)
    else
      super
    end
  end
end

# and use FooCompat, then when migration finished - revert to Foo and delete FooCompat
# is there a better way?


# Ruby 2 solution

module CompatLayer
  def digest
    if old?
      Digest::MD5.hexdigest(read_value)
    else
      super
    end
  end
end

class Foo
  prepend CompatLayer # <-- just remove this line later and you're done

  def digest
    Digest::SHA1.hexdigest(read_value)
  end
end