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.

	# First model: tag.rb
	# note that pomodori is a custom plural for pomodoro
	class Tag < ActiveRecord::Base
	has_and_belongs_to_many :pomodori
	end
	# Second model: pomodoro.rb
	# here is how to define a Rails 3 scope through the join table:
	class Pomodoro < ActiveRecord::Base
	has_and_belongs_to_many :tags
	scope :by_tag, lambda { |tag_text|
	joins("join pomodori_tags, tags").
	where('pomodori.id = pomodori_tags.pomodoro_id
	AND pomodori_tags.tag_id = tags.id
	AND tags.text = ?', tag_text)
	}
	end

view raw habtm_scope.rb hosted with ❤ by GitHub

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.