It’s been a while since I’ve updated my app on heroku. In between the mongoid version updated and configuration file (mongoid.yml) did change. That for me was the most obvious change, but then as well, there were a couple of things under the hood, that only showed up once I tried to deploy on heroku. The above error appeared and left me clueless for a moment, until I figured out, I had to define the ruby version 1.9.3 in my Gemfile:
source 'http://rubygems.org' ruby '1.9.3' gem 'rails', '~>3.2' gem 'jquery-rails' gem 'mongoid'
For who is interested, the new mongoid.yml looks like this in my project:
# set these environment variables on your prod server production: sessions: default: uri: <%= ENV['MONGOLAB_URL'] %> options: skip_version_check: true safe: true