Ruby：如何计算相对于另一条路径的路径？

比如说，我知道我要开始的绝对路径和我要尝试达到的绝对路径：

first = '/first/path'
second = '/second/path'

现在我想弄清楚如何构造一条相对于第一条的路径。例如：

# answer should be /first/path/../../second/path
path = second.get_path_relative_to(first)

我如何在Ruby中做这类事情？

使用：

require 'pathname'

first = Pathname.new '/first/path'
second = Pathname.new '/second/path'

relative = second.relative_path_from first
# ../../second/path

first + relative
# /second/path