Rails 3 scopes with HABTM (has and belongs to many) relations

There are already many posts about this, but maybe this simple example will help you understand this subject even better.

invalid option: --with-pg-dir=/opt/PostgreSQL/9.0

I'd bet a lot of ruby devs actually found themselves stuck in this problem. You checkout a github repo, you run a bundle install and = duh = a gem cannot install because of a missing library.
You're sure you've already installed the library or dependency or whatever, but in a different path from the standard one (in this example I'm talking about PostgreSQL installed via the graphical installer instead of the ubuntu apt repo); so you issue the command

gem install pg -v0.9.0 --with-pg-dir=/opt/PostgreSQL/9.0

And you get this error message: invalid option: --with-pg-dir=/opt/PostgreSQL/9.0

What's the problem? You need to separate options with another pair of dashes:

gem install pg -v0.9.0 -- --with-pg-dir=/opt/PostgreSQL/9.0

And everything will work.