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I recently tried to set up mongoid with a free mongohq account on heroku... This info is accurate as of 28 Feb, 2012.

For the impatient (I just want to make it work)

mongoid.yml

production:
  uri: <%= ENV['MONGOLAB_URI'] %>

Gemfile

gem 'bson', '1.3.1'
gem 'bson_ext', '1.3.1'
gem 'mongoid', '2.0.2'

For those who want more detail (I have a similar problem, how do I troubleshoot

To determine heroku environment variables, do this:

$ heroku config

and the results should contain a line like what follows:
...
MONGOLAB_URI        => mongodb://heroku_app928349384:lkfjgoierheourhgoeurhgoeuh@ds031087.mongolab.com:31087/heroku_appapp928349384
...

The important thing to note is what the left hand line says. Around the internet (like here on SO they will incorrectly name the ENV as MONGOHQ_URI. So then update mongoid.yml like this:

Additionally, it appears that different versions of mongoid only work with certain versions of mongodb. I started by digging around on google to find versions that seemed to be compatible.

Stack trace with wrong URI:
production:
  uri: <%= ENV['MONGOLAB_URI'] %>

Will yield something like

2012-02-28T12:49:46+00:00 app[web.1]: /app/.bundle/gems/ruby/1.9.1/gems/mongo-1.3.1/lib/mongo/connection.rb:518:in `connect': Failed to connect to a master node at localhost:27017 (Mongo::ConnectionFailure)
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/mongo-1.3.1/lib/mongo/connection.rb:656:in `setup'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/mongo-1.3.1/lib/mongo/connection.rb:101:in `initialize'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/mongo-1.3.1/lib/mongo/connection.rb:152:in `new'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/mongo-1.3.1/lib/mongo/connection.rb:152:in `from_uri'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/mongoid-2.0.2/lib/mongoid/config/database.rb:86:in `master'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/mongoid-2.0.2/lib/mongoid/config/database.rb:19:in `configure'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/mongoid-2.0.2/lib/mongoid/config.rb:114:in `from_hash'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/mongoid-2.0.2/lib/mongoid/config.rb:342:in `configure_databases'
2012-02-28T12:49:46+00:00 app[web.1]:  from (eval):2:in `from_hash'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/mongoid-2.0.2/lib/mongoid/railtie.rb:64:in `block in <class:Railtie>'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/initializable.rb:25:in `instance_exec'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/initializable.rb:25:in `run'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/initializable.rb:50:in `block in run_initializers'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/initializable.rb:49:in `run_initializers'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/initializable.rb:49:in `each'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/application.rb:134:in `initialize!'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/application.rb:77:in `method_missing'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/config/environment.rb:5:in `<top (required)>'
2012-02-28T12:49:46+00:00 app[web.1]:  from <internal:lib/rubygems/custom_require>:29:in `require'
2012-02-28T12:49:46+00:00 app[web.1]:  from <internal:lib/rubygems/custom_require>:29:in `require'
2012-02-28T12:49:46+00:00 app[web.1]:  from config.ru:3:in `block (3 levels) in <main>'
2012-02-28T12:49:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:23:in `eval'
2012-02-28T12:49:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:23:in `block (3 levels) in <main>'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:46:in `instance_eval'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:46:in `initialize'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:63:in `new'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:63:in `map'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:46:in `initialize'
2012-02-28T12:49:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:18:in `block (2 levels) in <main>'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:46:in `instance_eval'
2012-02-28T12:49:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:11:in `block in <main>'
2012-02-28T12:49:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:11:in `new'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:46:in `instance_eval'
2012-02-28T12:49:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:46:in `initialize'
2012-02-28T12:49:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:1:in `new'
2012-02-28T12:49:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:1:in `<main>'
2012-02-28T12:49:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/lib/rack/adapter/loader.rb:36:in `eval'
2012-02-28T12:49:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/lib/rack/adapter/loader.rb:36:in `load'
2012-02-28T12:49:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/lib/thin/controllers/controller.rb:175:in `load_rackup_config'
2012-02-28T12:49:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/lib/thin/controllers/controller.rb:65:in `start'
2012-02-28T12:49:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/lib/thin/runner.rb:177:in `run_command'
2012-02-28T12:49:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/lib/thin/runner.

2012-02-28T12:49:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/bin/thin:6:in `<top (required)>'
2012-02-28T12:49:46+00:00 app[web.1]:  from /usr/ruby1.9.2/bin/thin:19:in `load'
2012-02-28T12:49:46+00:00 app[web.1]:  from /usr/ruby1.9.2/bin/thin:19:in `<main>'
2012-02-28T12:49:47+00:00 heroku[web.1]: Process exited with status 1
2012-02-28T12:49:47+00:00 heroku[web.1]: State changed from starting to crashed

Stack trace with wrong version of mongo/mongoid/bson :

Starting process with command `thin -p 4644 -e production -R /home/heroku_rack/heroku.ru start`
2012-02-27T22:50:46+00:00 app[web.1]: /app/app/models/expression.rb:2:in `<class:Expression>': uninitialized constant Expression::Mongoid (NameError)
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/app/models/expression.rb:1:in `<top (required)>'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/engine.rb:138:in `block (2 levels) in eager_load!'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/engine.rb:137:in `each'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/engine.rb:137:in `block in eager_load!'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/engine.rb:135:in `eager_load!'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/engine.rb:135:in `each'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/application.rb:108:in `eager_load!'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/application/finisher.rb:41:in `block in <module:Finisher>'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/initializable.rb:25:in `instance_exec'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/initializable.rb:25:in `run'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/initializable.rb:50:in `block in run_initializers'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/initializable.rb:49:in `each'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/initializable.rb:49:in `run_initializers'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/application.rb:134:in `initialize!'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.8/lib/rails/application.rb:77:in `method_missing'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/config/environment.rb:5:in `<top (required)>'
2012-02-27T22:50:46+00:00 app[web.1]:  from <internal:lib/rubygems/custom_require>:29:in `require'
2012-02-27T22:50:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:23:in `eval'
2012-02-27T22:50:46+00:00 app[web.1]:  from <internal:lib/rubygems/custom_require>:29:in `require'
2012-02-27T22:50:46+00:00 app[web.1]:  from config.ru:3:in `block (3 levels) in <main>'
2012-02-27T22:50:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:23:in `block (3 levels) in <main>'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:46:in `instance_eval'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:46:in `initialize'
2012-02-27T22:50:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:18:in `block (2 levels) in <main>'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:46:in `initialize'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:63:in `new'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:63:in `map'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:46:in `instance_eval'
2012-02-27T22:50:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:11:in `new'
2012-02-27T22:50:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:11:in `block in <main>'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:46:in `instance_eval'
2012-02-27T22:50:46+00:00 app[web.1]:  from /app/.bundle/gems/ruby/1.9.1/gems/rack-1.2.5/lib/rack/builder.rb:46:in `initialize'
2012-02-27T22:50:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:1:in `new'
2012-02-27T22:50:46+00:00 app[web.1]:  from /home/heroku_rack/heroku.ru:1:in `<main>'
2012-02-27T22:50:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/lib/rack/adapter/loader.rb:36:in `eval'
2012-02-27T22:50:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/lib/thin/controllers/controller.rb:175:in `load_rackup_config'
2012-02-27T22:50:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/lib/rack/adapter/loader.rb:36:in `load'
2012-02-27T22:50:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/lib/thin/controllers/controller.rb:65:in `start'
2012-02-27T22:50:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/lib/thin/runner.rb:177:in `run_command'
2012-02-27T22:50:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/bin/thin:6:in `<top (required)>'
2012-02-27T22:50:46+00:00 app[web.1]:  from /usr/ruby1.9.2/lib/ruby/gems/1.9.1/gems/thin-1.2.6/lib/thin/runner.rb:143:in `run!'
2012-02-27T22:50:46+00:00 app[web.1]:  from /usr/ruby1.9.2/bin/thin:19:in `load'
2012-02-27T22:50:46+00:00 app[web.1]:  from /usr/ruby1.9.2/bin/thin:19:in `<main>'
2012-02-27T22:50:47+00:00 heroku[web.1]: Process exited with status 1
2012-02-27T22:50:47+00:00 heroku[web.1]: State changed from starting to crashed
2012-02-27T22:50:48+00:00 heroku[router]: Error H10 (App crashed) -> POST newsfilter.heroku.com/_heroku/console dyno= queue= wait= service= status=503 bytes=

 

 

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