Bare bones rails app with the following gems:

gem 'rails',                         '3.2.6'
gem 'mongoid',                       '2.4.10'
gem 'bson_ext',                      '1.6.2'
gem 'money-rails',                   '0.4.0'

Create a model

class Transaction
  include Mongoid::Document
  include Mongoid::Timestamps

  field :amount, type: Money
end

Fails in concole

♫  ~/code/rm-test$ rails c
Loading development environment (Rails 3.2.6)
irb(main):001:0> Transaction.new
NoMethodError: undefined method `instantiate' for Money:Class
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.4.10/lib/mongoid/fields.rb:284:in `add_field'
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.4.10/lib/mongoid/fields.rb:195:in `field'
  from /Users/barmstrong/code/rm-test/app/models/transaction.rb:5:in `<class:Transaction>'
  from /Users/barmstrong/code/rm-test/app/models/transaction.rb:1:in `<top (required)>'
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activesupport-3.2.6/lib/active_support/dependencies.rb:469:in `load'
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activesupport-3.2.6/lib/active_support/dependencies.rb:469:in `block in load_file'
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activesupport-3.2.6/lib/active_support/dependencies.rb:639:in `new_constants_in'
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activesupport-3.2.6/lib/active_support/dependencies.rb:468:in `load_file'
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activesupport-3.2.6/lib/active_support/dependencies.rb:353:in `require_or_load'
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activesupport-3.2.6/lib/active_support/dependencies.rb:502:in `load_missing_constant'
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activesupport-3.2.6/lib/active_support/dependencies.rb:192:in `block in const_missing'
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activesupport-3.2.6/lib/active_support/dependencies.rb:190:in `each'
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activesupport-3.2.6/lib/active_support/dependencies.rb:190:in `const_missing'
  from (irb):1
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.6/lib/rails/commands/console.rb:47:in `start'
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.6/lib/rails/commands/console.rb:8:in `start'
  from /Users/barmstrong/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.6/lib/rails/commands.rb:41:in `<top (required)>'
  from script/rails:6:in `require'
  from script/rails:6:in `<main>'irb(main):002:0> exit
♫  ~/code/rm-test$