I was having terrible difficulty getting Merb to work with RSpec. Turns out that Merb now uses Bundler, and if you don't edit your Gemfile to include rspec it bails on you.
sam@shiny-dev:~/Projects/mojo$ rake db:migrate
Merb root at: /home/sam/Projects/mojo
/home/sam/.rvm/gems/ruby-1.9.1-p378@merb/gems/dm-validations-1.0.0/lib/dm-validations.rb:33: warning: already initialized constant OrderedHash
Loading init file from ./config/init.rb
no such file to load -- spec/rake/spectask
(See full trace by running task with --trace)
This was solved by adding the line:
gem "rspec", :require => "spec"
to the bottom of the Gemfile in the project root.