Heroku & MongoHQ: ActionView::Template::Error (The operation: #<Moped::Protocol::Commands::Authenticate failed with error "auth fails")
===

I have a Rails 3 app with MongoID 3 deployed successfully to Heroku (using MongoHQ) -- but then something happened (cannot pinpoint what exactly it was) and the build started crashing on Heroku.

So this is the error I get: 

**TLDR:** `Moped::Errors::AuthenticationFailure (The operation: #<Moped::Protocol::Commands::Authenticate ... failed with error "auth fails")`

What makes this more difficult to debug is that it's working just fine on localhost.
Now I'm running out of ideas what this could be.

	app[web.1]: >> Thin web server (v1.4.1 codename Chromeo)
	app[web.1]: >> Maximum connections set to 1024
	app[web.1]: >> Listening on 0.0.0.0:58731, CTRL+C to stop
	heroku[web.1]: State changed from starting to up
	app[web.1]: 
	app[web.1]: 
	app[web.1]: Started GET "/" for 212.86.9.90 at 2012-07-31 08:08:07 +0000
	heroku[router]: GET cool-name-123.herokuapp.com/ dyno=web.1 queue=0 wait=0ms service=171ms status=200 bytes=1286
	heroku[router]: GET cool-name-123.herokuapp.com/assets/application-8e7bfeeffc9291864e5b42d908c2fdda.css dyno=web.1 queue=0 wait=0ms service=11ms status=200 bytes=92524
	heroku[router]: GET cool-name-123.herokuapp.com/assets/application-aa557bde70f1236cdf90c913043c4382.js dyno=web.1 queue=0 wait=0ms service=16ms status=200 bytes=122836
	heroku[router]: GET cool-name-123.herokuapp.com/favicon.ico dyno=web.1 queue=0 wait=0ms service=3ms status=200 bytes=0
	app[web.1]: 
	app[web.1]: 
	app[web.1]: Started GET "/originals/new" for 212.86.9.90 at 2012-07-31 08:08:11 +0000
	heroku[router]: GET cool-name-123.herokuapp.com/originals/new dyno=web.1 queue=0 wait=0ms service=31ms status=200 bytes=1808
	app[web.1]: 
	app[web.1]: 
	app[web.1]: Started POST "/originals" for 212.86.9.90 at 2012-07-31 08:08:34 +0000
	app[web.1]: 
	app[web.1]: Moped::Errors::AuthenticationFailure (The operation: #<Moped::Protocol::Commands::Authenticate
	app[web.1]:   @length=154
	app[web.1]:   @request_id=3
	app[web.1]:   @response_to=0
	app[web.1]:   @op_code=2004
	app[web.1]:   @flags=[]
	app[web.1]:   @full_collection_name="app123.$cmd"
	app[web.1]:   @limit=-1
	app[web.1]:   @skip=0
	app[web.1]:   @selector={:authenticate=>1, :user=>"heroku", :nonce=>"xyz", :key=>"xyz"}
	heroku[router]: POST cool-name-123.herokuapp.com/originals dyno=web.1 queue=0 wait=0ms service=1486ms status=500 bytes=643
	app[web.1]:   @fields=nil>
	app[web.1]: failed with error "auth fails"):
	app[web.1]:   app/controllers/originals_controller.rb:19:in `block in create'
	app[web.1]:   app/controllers/originals_controller.rb:18:in `create'
	app[web.1]: 
	app[web.1]: 
	heroku[router]: GET cool-name-123.herokuapp.com/favicon.ico dyno=web.1 queue=0 wait=0ms service=3ms status=304 bytes=0
	app[web.1]: 
	app[web.1]: 
	app[web.1]: Started GET "/" for 212.86.9.90 at 2012-07-31 08:09:40 +0000
	heroku[router]: GET cool-name-123.herokuapp.com/ dyno=web.1 queue=0 wait=0ms service=11ms status=304 bytes=0

I've recently struggled with Heroku, and as a result I think I should have the correct settings for the choice of db drivers & service (using the new yml scheme + Ruby 1.9.3 in the Gemfile).

When I run `heroku config --app app_name` I get

	DATABASE_URL        => postgres://rhalppyjrb:xyz@ec2-xyz.compute-1.amazonaws.com/rhalppyjrb
	GEM_PATH            => vendor/bundle/ruby/1.9.1
	LANG                => en_US.UTF-8
	MONGOHQ_URL         => mongodb://heroku:xyz@flame.mongohq.com:27054/app123
	PATH                => bin:vendor/bundle/ruby/1.9.1/bin:/usr/local/bin:/usr/bin:/bin
	RACK_ENV            => production
	RAILS_ENV           => production
	SHARED_DATABASE_URL => postgres://rhalppyjrb:xyz@ec2-123.compute-1.amazonaws.com/rhalppyjrb

My mongoid.yml looks like this: 

	production:
	  sessions:
	    default:
	          uri: <%= ENV['MONGOHQ_URL'] %>
	          options:
	            consistency: :strong
	            skip_version_check: true
	            safe: true

Gemfile is like this:

	source 'https://rubygems.org'

	gem 'bundler', '1.2.0.rc'

	ruby '1.9.3'

	gem 'rails', '3.2.6'
	gem 'thin'
	gem 'rmagick', :require => 'RMagick'
	gem "carrierwave-mongoid", :git => "git://github.com/jnicklas/carrierwave-mongoid.git", :branch => "mongoid-3.0", :require => 'carrierwave/mongoid'

	group :assets do
	  gem 'sass-rails',   '~> 3.2.3'
	  gem 'coffee-rails', '~> 3.2.1'
	  gem 'uglifier', '>= 1.0.3'
	  gem "therubyracer", :platform => :ruby
	end

	gem 'jquery-rails'
	gem "haml", ">= 3.1.6"
	gem "mongoid", ">= 3.0.3"
	gem "devise", ">= 2.1.2"
	gem "devise_invitable", ">= 1.0.2"
	gem "cancan", ">= 1.6.8"
	gem "rolify", ">= 3.1.0"
	gem "bootstrap-sass", ">= 2.0.4.0"
	gem "simple_form"

	group :development do
	  gem "guard", ">= 0.6.2"
	  gem 'rb-fsevent'
	  gem 'growl'
	  gem "haml-rails", ">= 0.3.4"
	  gem "guard-bundler", ">= 0.1.3"
	  gem "guard-rails", ">= 0.0.3"
	  gem "guard-livereload", ">= 0.3.0"
	  gem "guard-rspec", ">= 0.4.3"
	end

	group :test do
	  gem "database_cleaner", ">= 0.8.0"
	  gem "mongoid-rspec", "1.4.6"
	  gem "email_spec", ">= 1.2.1"
	end

	group :development, :test do
	  gem "factory_girl_rails", ">= 3.5.0"
	  gem "rspec-rails", ">= 2.11.0"
	end