The problems happens after you’ve pushed your app up, when you’re trying to run your rake db:migrate.
So I found the solution in one of the posts over here: at stackoverflow, … so if this doesn’t work for you, try one of the other answers (shrug).
Basically, I had to use an older version of rake. So in my Gemfile, I added:
# Hack to workaround Heroku 'uninitialized constant Rake::DSL' # gem "rake", "0.8.7"
Then a few commands to get ourselves up to date:
$ bundle update rake
$ git commit -a -m "Use 0.8.7 of rake"
And then of course re-run our db:migrate:
$ heroku rake db:migrate