Ruby gem loaded but still gives "uninitialized constant" - mongodb

Question

I don't understand this output:
$ rails c
Loading development environment (Rails 3.2.14)
1.9.3p392 :001> require 'rubygems'
=> false
1.9.3p392 :002 > require 'mongo'
=> false
1.9.3p392 :003 > include Mongo
=> Object
1.9.3p392 :004 > Mongo::VERSION
=> "1.6.0"
1.9.3p392 :005 > c = MongoClient.new
NameError: uninitialized constant MongoClient
from (irb):5
Clearly bundler is running correctly and loading the MongoDB driver gem "mongo" (version 1.6, which should be fine).
Yet when I try to reference MongoClient according to the docs, I get "uninitialized constant." The same happens if I try to reference it as Mongo::MongoClient.new
What am I missing here?

Answer 1

From your console output, you are using MongoDB Ruby driver version 1.6.0.
The corresponding API docs can be found here.
http://api.mongodb.org/ruby/1.6.0/
Version 1.6.0 uses the old Mongo::Connection class to connect to MongoDB.
The MongoClient class is new as of version 1.8.0, and so is not available in any older version.
Please use the matching API and documentation.
Please consider upgrading to newer versions of the mongo_mapper (0.12.0) and mongo (1.9.2) gems.