Unsuccessful with using mongodb with rails

I have had a frustrating time trying to get mongodb to work with Rails
(works just fine with a plain Ruby app).

cannot load such file – mongo

The error is traced to the code line <requires ‘Mongo’>.

bundle initialize shows I have it all. Any ideas?

Thanks,
Shiv

$ bundle install

Using rake (9.2.2)
Using i18n (0.6.0)
Using multi_json (1.3.6)
Using activesupport (3.2.8)
Using builder (3.0.0)
Using activemodel (3.2.8)
Using erubis (2.7.0)
Using journey (1.0.4)

Using i18n (0.6.0)
Using multi_json (1.3.6)
Using activesupport (3.2.8)
Using builder (3.0.0)
Using activemodel (3.2.8)
Using erubis (2.7.0)
Using journey (1.0.4)
Using rack (1.4.1)
Using rack-cache (1.2)
Using rack (1.4.1)
Using rack-cache (1.2)

Using polyglot (0.3.3)
Using treetop (1.4.10)
Using mail (2.4.4)
Using actionmailer (3.2.8)
Using arel (3.0.2)
Using tzinfo (0.3.33)
Using activerecord (3.2.8)
Using activeresource (3.2.8)
Using bson (1.6.4)
Using coffee-script-source (1.3.3)
Using execjs (1.4.0)
Using coffee-script (2.2.0)
Using rack-ssl (1.3.2)
Using json (1.7.5)
Using rdoc (3.12)
Using thor (0.16.0)
Using railties (3.2.8)
Using coffee-rails (3.2.2)
Using jquery-rails (2.1.1)
Using mongo (1.6.4)
Using plucky (0.5.1)
Using mongo_mapper (0.11.2)
Using bundler (1.1.5)
Using rails (3.2.8)
Using sass (3.2.1)
Using sass-rails (3.2.5)
Using uglifier (1.2.7)

Your bundle is complete!