Eu tive vários problemas de bundler/gem com um novo projeto Rails 3.2.3 seguido pelo seu Gemfile. Consegui superar isso removendo o Gemfile.lock e executando novamente a 'instalação do pacote'. O log do que fiz está anexado e inclui a excisão do ActiveRecord. Espero que isso ajude.
$ ruby -v
ruby 1.9.3p125 (2012-02-16 revision 34643) [x86_64-darwin11.3.0]
$ rails new free-11479-mongoid-rails-g
$ cd free-11479-mongoid-rails-g
Gemfile as per user
$ bundle install
You have requested:
json ~> 1.6.3
The bundle currently has json locked at 1.7.1.
Try running `bundle update json`
$ bundle update json
Bundler could not find compatible versions for gem "multi_json":
In Gemfile:
rabl (~> 0.5.1) ruby depends on
multi_json (~> 1.0.3) ruby
guard-jasmine (>= 0) ruby depends on
multi_json (1.3.4)
$ rm Gemfile.lock
$ bundle install
$ rails g mongoid:config
create config/mongoid.yml
---------
Assuming that you want to excise ActiveRecord...
dereference ActiveRecord
config/application.rb
#config.active_record.whitelist_attributes = true
config/environments/development.rb
#config.active_record.mass_assignment_sanitizer = :strict
#config.active_record.auto_explain_threshold_in_seconds = 0.5
config/environments/development.rb
#config.active_record.mass_assignment_sanitizer = :strict
test/test_helper.rb
#fixtures :all
$ rm database.yml
config/application.rb
#require 'rails/all'
require "action_controller/railtie"
require "action_mailer/railtie"
require "active_resource/railtie"
require "rails/test_unit/railtie"
$ rake test # passes
