I actually recommend that people spend only about an hour or two on Ruby before getting into Rails. Ruby is very intuitive and the few things that aren't can be temporarily memorized until they make sense.
My roadmap would be:
1) sinatra hello world
2) sinatra hello world w/ erb template
3) deploy it to heroku
4) do something with a form (without form helpers)
It will to a developer. This is aimed at newbies. When you've been a developer for a while, so many skills become automatic to the point that they are invisible to you and you forget that newbies have to learn them.
Beginning programmers still struggle with proper indentation, for example. That's something they have to learn.
My roadmap would be:
1) sinatra hello world
2) sinatra hello world w/ erb template
3) deploy it to heroku
4) do something with a form (without form helpers)
5) do something with sessions
6) do something with datamapper
7) who needs rails?