Rails 4 + Mongoid setup (updated)

Note:

You should always create your Rails application using the flag --skip-active-record, but if your forget it or the damage is done, you can use this workaround.

Update:

Mongoid 4.0.0 has been released!

In your Gemfile replace the following lines:

# Use sqlite3 as the database for Active Record
gem 'sqlite3'

With:

gem 'mongoid', '~> 4.0.0'
gem 'bson_ext'

Then execute:

bundle update
bundle exec rails g mongoid:config

Now, in config/application.rb file replace the line:

require 'rails/all'

With:

require "action_controller/railtie"
require "action_mailer/railtie"
# require "active_resource/railtie"
require "sprockets/railtie"

If you want to use ActiveResource, you will need to require the gem and then comment out the commented line.

And, in config/environments/development.rb, comment the line:

# config.active_record.migration_error = :page_load