Mongoid 3 + Heroku (MongoHQ) cause Moped::Errors::OperationFailure - ruby-on-rails

Question

With Rails 3, after upgrading from Mongoid 2.x to Mongoid 3.x, my Heroku + MongoHQ setup stopped working. Funny thing is, that my development & test frameworks and my whole test suite passes just fine.
I suspect the problem is with my mongoid.yml file, but I've tried searching the docs, google & stackoverflow, and used all the suggested formats, including this: heroku mongohq and mongoid Mongo::ConnectionFailure or actually this: https://gist.github.com/2900804
UPDATED July 16th: This is how my mongoid.yml file looks, after trying multiple things + After what Jason from MongoHQ suggested:
development:
sessions:
default:
database: development
hosts:
- localhost:27017
test:
sessions:
default:
database: test
hosts:
- localhost:27017
production:
sessions:
default:
uri: <%= ENV['MONGOHQ_URL'] %>
options:
skip_version_check: true
safe: true
(to my understanding, it is essentially the same as the one in the links above, except that it uses the uri; I tried the other way, splitting the MONGOHQ_URL into the separate fields as well, but it didn't help)
I've tried setting mongoid as 3.0.0rc and leaving the version blank in my Gemfile. Using the github version failed due to HTTPS certificates or something, so I didn't bother trying it multiple times.
What the action controller says is this:
Moped::Errors::OperationFailure in Home#index
Showing /app/app/views/home/index.html.haml where line #2 raised:
The operation: #<Moped::Protocol::Command
#length=68
#request_id=4
#response_to=0
#op_code=2004
#flags=[:slave_ok]
#full_collection_name=".$cmd"
#skip=0
#limit=-1
#selector={:count=>:posts, :query=>{}}
#fields=nil>
failed with error "db assertion failure"
and when I run
heroku run console
.. I get this:
irb(main):052:0> Location.create!
NoMethodError: undefined method `[]' for nil:NilClass
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/node.rb:74:in `block in command'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/node.rb:522:in `[]'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/node.rb:522:in `block (3 levels) in flush'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/node.rb:521:in `map'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/node.rb:521:in `block (2 levels) in flush'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/node.rb:113:in `ensure_connected'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/node.rb:517:in `block in flush'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/node.rb:532:in `logging'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/node.rb:516:in `flush'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/node.rb:505:in `process'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/node.rb:70:in `command'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/node.rb:356:in `refresh'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/cluster.rb:101:in `block in refresh'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/cluster.rb:114:in `each'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/cluster.rb:114:in `refresh'
from /app/vendor/bundle/ruby/1.9.1/gems/moped-1.1.1/lib/moped/cluster.rb:67:in `nodes'
... 15 levels...
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:405:in `__run_callback'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:385:in `_run_save_callbacks'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:81:in `run_callbacks'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.1/lib/mongoid/callbacks.rb:98:in `run_callbacks'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.1/lib/mongoid/persistence/insertion.rb:23:in `prepare'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.1/lib/mongoid/persistence/operations/insert.rb:26:in `persist'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.1/lib/mongoid/persistence.rb:50:in `insert'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.1/lib/mongoid/persistence.rb:251:in `block in create!'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.1/lib/mongoid/threaded/lifecycle.rb:173:in `_creating'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.1/lib/mongoid/persistence.rb:249:in `create!'
from (irb):52
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.6/lib/rails/commands/console.rb:47:in `start'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.6/lib/rails/commands/console.rb:8:in `start'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.6/lib/rails/commands.rb:41:in `<top (required)>'
from script/rails:6:in `require'
from script/rails:6:in `<main>'irb(main):053:0>
I think I've googled everything, run through multiple github mongoid issues, read the documentation multiple times ... and I'm running out of ideas here.
Anything come to mind I should try next?
UPDATE July 16th: this is what Heroku says when I git push heroku master (After doing what Jason from MongoHQ suggested):
git push heroku master
Counting objects: 7, done.
Delta compression using up to 16 threads.
Compressing objects: 100% (4/4), done.
Writing objects: 100% (4/4), 372 bytes, done.
Total 4 (delta 3), reused 0 (delta 0)
-----> Heroku receiving push
-----> Ruby/Rails app detected
-----> Installing dependencies using Bundler version 1.2.0.pre
Running: bundle install --without development:test --path vendor/bundle --binstubs bin/ --deployment
Using rake (0.9.2.2)
Using i18n (0.6.0)
Using multi_json (1.3.6)
Using activesupport (3.2.6)
Using builder (3.0.0)
# ... shortened this #
Using libv8 (3.3.10.4)
Using moped (1.1.2)
Using origin (1.0.4)
Using mongoid (3.0.0.rc)
Using omniauth (1.1.0)
Using quimby (0.4.5)
Using bundler (1.2.0.pre)
Using rails (3.2.6)
Using therubyracer (0.10.1)
Using thin (1.3.1)
Your bundle is complete! It was installed into ./vendor/bundle
Cleaning up the bundler cache.
-----> Writing config/database.yml to read from DATABASE_URL
-----> Preparing app for Rails asset pipeline
Running: rake assets:precompile
Asset precompilation completed (31.24s)
-----> Rails plugin injection
Injecting rails_log_stdout
Injecting rails3_serve_static_assets
-----> Discovering process types
Procfile declares types -> (none)
Default types for Ruby/Rails -> console, rake, web, worker
-----> Compiled slug size is 18.7MB
-----> Launching... done, v92
http://xxxxx.herokuapp.com deployed to Heroku
To git#heroku.com:xxxxx.git
b2d97xy..7b0aczy master -> master
UPDATE 2 July 16th: did as Jason & MrKurt said (first forgot to run bundle update mongoid, but that didn't help either). Now the error changed, though:
NoMethodError in Home#index
Showing /app/app/views/home/index.html.haml where line #2 raised:
undefined method `[]' for nil:NilClass
Extracted source (around line #2):
1: %h1 Most recent posts
2: - if #posts.length > 0

Answer 1

You will need to use Ruby 1.9.3. See Mongoid documentation here and Heroku documentation here on how to use 1.9.3.
I was also getting similar errors, and I changed my gemfile to include ruby '1.9.3' and everything worked as expected again.
Edit:
#herb pointed out that you need the latest version of bundler for this to work (gem install bundler --pre). Otherwise, ruby '1.9.3' in your Gemfile will not work.

Answer 2

Try adding an "options" area to the default session and add "skip_version_check: true" as one of the options.
Jason
MongoHQ

Answer 3

As a workaround, hardcoding the uri from heroku config seems to do the trick.
ex:
production:
sessions:
default:
uri: "mongodb://heroku:xxxx#domain.mongohq.com:000/appyyyy"
options:
skip_version_check: true
safe: true
I'm guessing the nilclass error was because <%= ENV['MONGOHQ_URL'] %> was returning nil