无法在Ruby脚本中加载相对文件

文件组织如下：

- myScriptProject
  - solutions.ru
  - main.ru

solution.ru

def test_function
  puts 'Hello World'
end

main.rb

#!/Users/myusername/.rbenv/shims/ruby

require './solution' # or require_relative 'solution'

test_function

我正在尝试运行脚本：

$ cd myScriptProject/
$ ruby main.ru

然后我收到了一条LoadError消息

Traceback (most recent call last):
        2: from test.ru:3:in `<main>'
        1: from /Users/myusername/.rbenv/versions/2.6.3/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
/Users/myusername/.rbenv/versions/2.6.3/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require': cannot load such file -- ./solution (LoadError)

---

Ruby的脚本扩展是

rb

而不是

ru

$ which ruby
$ /Users/myusername/.rbenv/shims/ruby