If you are working with Rails 3 and Mongoid, you're likely to upgrade to [`mongoid-2.0.0.beta6`][1]. That's okay, but you will run into a few problems. Among others, one will be:
The first thing you need to do is make sure you use the right version of `bson_ext`. Beta 6 requires you to run `bson_ext-1.0.1` or you'll get the `OrderedHash` error. Okay, with that out of the way, let's focus on the MongoDB errors.
The problem is that Mongoid is accessed/used before it is properly initialized. To resolve this issue, add the following line to your other requires at the top of `config/application.rb`.
With that, you initialize Mongoid correctly. Hope it helps.
Update: I also ran into `keys must be strings or symbols` errors. This is now a _known issue_ with `mongoid-2.0.0.beta6` and has been fixed in `master`. If you are using Bundler (you are, aren't you?) then you can use the master branch instead of the gem: