I m going by this from reading the source, so let me know if it actually worked. :)

The rails command that generates the application template now has an option -O, which tells it to skip ActiveRecord.

If you don t feel like rerunning rails, you should check the following in your existing app:

• Check that your config/application.rb doesn t have require rails/all or require "active_record/railtie". Instead, for a standard Rails setup without ActiveRecord, it should have only the following requires:

  require File.expand_path( ../boot , __FILE__)
  
  require "action_controller/railtie"
  require "action_mailer/railtie"
  require "active_resource/railtie"
  require "rails/test_unit/railtie"
  require "sprockets/railtie"
  
  # Auto-require default libraries and those for the current Rails environment. 
  Bundler.require :default, Rails.env
  

• If, in config/application.rb, you are using the config.generators section, make sure it doesn t have the line g.orm :active_record. You can set this explicitly to nil, if you want, but this should be the default when g.orm is completely omitted.

• Optional, but in your Gemfile, remove the gem line that loads the module for your database. This could be the line gem "mysql" for example.

Rails 4

I was looking for how to disable it in rails 4 and only found this answer which no longer works in rails 4. So this is how you can do it in rails 4 (tested in RC1).

In a new project

rails new YourProject --skip-active-record

In an existing project

• In your Gemfile, remove the database driver gem, e.g. gem sqlite3 or gem pg .

• In config/application.rb, replace require rails/all with

  require "action_controller/railtie"
  require "action_mailer/railtie"
  require "sprockets/railtie"
  require "rails/test_unit/railtie"
  

• In config/environments/development.rb, remove or comment out config.active_record.migration_error = :page_load

• Potentially you have to remove active_record helpers from the spec_helper (via VenoM in the comments)

• Potentially you have to remove the ConnectionManagement middleware (seems to be the case with unicorn): config.app_middleware.delete "ActiveRecord::ConnectionAdapters::ConnectionManagement" (via https://stackoverflow.com/a/18087332/764342)

I hope this helps others looking for how to disable ActiveRecord in Rails 4.

For a new rails app, you can have it exclude active record by specifying the --skip-active-record parameter. Eg:

rails new appname --skip-active-record

If you generated a new project using Rails 3.2, you will also need to comment out:

config.active_record.mass_assignment_sanitizer = :strict

and

config.active_record.auto_explain_threshold_in_seconds = 0.5

in your development.rb file.

All of the above are true. The one more thing which I had to do in rails 3.1 is to comment out

config.active_record.identity_map = true

in config/application.rb.

If you re running rspec, you also need to remove (in spec_helper):

  # Remove this line if you re not using ActiveRecord or ActiveRecord fixtures
  config.fixture_path = "#{::Rails.root}/spec/fixtures"

and remove

  # If you re not using ActiveRecord, or you d prefer not to run each of your
  # examples within a transaction, remove the following line or assign false
  # instead of true.
  config.use_transactional_fixtures = true

Also comment out

# config/application.rb    
config.active_record.whitelist_attributes = true

(noted on rails 3.2.13)