I'm upgrading to Rails 3.2, and running rake db:migrate gives me several errors of the form:
DEPRECATION WARNING: You have Rails 2.3-style plugins in vendor/plugins! Support for these plugins will be removed in Rails 4.0. Move them out and bundle them in your Gemfile, or fold them in to your app as lib/myplugin/* and config/initializers/myplugin.rb. See the release notes for more on this: http://weblog.rubyonrails.org/2012/01/04/rails-3-2-0-rc2-has-been-released. (called from at /app/Rakefile:7)
What's perplexing is that my vendor/plugins directory is empty -- is there another plugins directory that it's referencing?
Are you using Heroku?
Heroku will inject plugins in Rails 3.x applications .. To avoid this injection in Rails 3, include the rails_12factor gem in your application. (Heroku Ruby Support 2013-10-26)
The rails_12factor gem is also required in rails 4.
If this gem is not present in your application, you will receive a warning while deploying, and your assets and logs will not be functional. (Rails 4 on Heroku 2013-10-26)
As recently as 2013-08, heroku always injected plugins in rails 3 apps, even apps with the recommended gems. This was an issue with the ruby buildpack, and was fixed by PR 11, merged on 2013-08-06.