Disable ActiveRecord for Rails 4

If you are creating a new application, you can use -O to skip ActiveRecord:

rails new my_app -O

For existing applications:

1. Remove database adapter gems from your Gemfile (mysql2, sqlite3, etc.)

2. Change your config/application.rb

Remove require 'rails/all line and require frameworks (among those available in your rails version, the list varies, do not just copy) you want to use, for example:

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

Remove config.active_record.raise_in_transactional_callbacks = true from config/application.rb

3. Delete your config/database.yml file, db/schema.rb and migrations (if any)

4. Delete migration check in test/test_helper.rb

5. Delete any ActiveRecord configuration from your config/environments files (this is what is causing your error)

This is all you need to do for an empty Rails app. If you run into problems caused by your existing code, stack trace should give you sufficient information on what you need to change. You might for example have some ActiveRecord configuration in your initializers.

Since this is still the first hit when searching Google for disabling active record for Rails 5, I'll add this here:

For Rails 5

Do all the steps in @mechanicalfish answer, but also remove the line

Rails.application.config.active_record.belongs_to_required_by_default = true

from

config/initializers/new_framework_defaults.rb

Hi this is what the default rails new new_app -O gives

require "rails"
# Pick the frameworks you want: 
require "active_model/railtie" 
require "active_job/railtie"
# require "active_record/railtie" 
require "action_controller/railtie" 
require "action_mailer/railtie" 
require "action_view/railtie" 
require "sprockets/railtie" 
require "rails/test_unit/railtie"

inside your config/application.rb

Additionally, it comes without database.yml and NO db/migrate/* and schema.rb