Rake 0.9.2, Rails 3.0.8 rc1: undefined method `prerequisites' for nil:NilClass

run rake db:migrate error

Rails 3.1 and Rspec-rails < 2.6 isn't compatible: https://github.com/rspec/rspec-rails/blob/master/features/RailsVersions.md.

upgrade to rspec-rails version 2.6.1.beta1 fixed it.

 solution:

If you haven't specified a version for rspec-rails in your Gemfile, just run bundle update.
If you have, upgrade it to 2.6.1 and run bundle update.