How to configure Ruby on Rails with no database?

For an existing Rails 4-7 project, in your config/application.rb file you have the following line:

require 'rails/all' # or `require "rails"' in newer versions

(As reference that line is loading this file)
So instead of load ALL, you must to load each library separately as follows:

# active_record is what we're not going to use it, so comment it "just in case"
# require "active_record/railtie" 

# This is not loaded in rails/all but inside active_record so add it if
# you want your models work as expected
require "active_model/railtie" 
# And now the rest
require "action_controller/railtie"
require "action_mailer/railtie"
require "action_view/railtie"
require "active_job/railtie" # Only for Rails >= 4.2
require "action_cable/engine" # Only for Rails >= 5.0
require "sprockets/railtie" # Deprecated for Rails >= 7, so add it only if you're using it
require "rails/test_unit/railtie"

# All these depend on active_record, so they should be excluded also
# require "action_text/engine" # Only for Rails >= 6.0
# require "action_mailbox/engine" # Only for Rails >= 6.0
# require "active_storage/engine" # Only for Rails >= 5.2

Keep an eye to the comments to know what to load regarding your Rails version.
Also check the following files (in case you have them) and comment the following lines:

# package.json
"@rails/activestorage": "^6.0.0",

# app/javascript/packs/application.js
require("@rails/activestorage").start()

# bin/setup
system! 'bin/rails db:prepare'

# config/environments/development.rb
config.active_storage.service = :local # For Rails >= 5.2
config.active_record.migration_error = :page_load
config.active_record.verbose_query_logs = true

# config/environments/test.rb
config.active_storage.service = :test # For Rails >= 5.2

# config/environments/production.rb
config.active_storage.service = :local # For Rails >= 5.2
config.active_record.dump_schema_after_migration = false

# spec/rails_helper.rb
ActiveRecord::Migration.maintain_test_schema!

# test/test_helper.rb
fixtures :all # In case you're using fixtures

# Only for Rails >= 5.0
#config/initializers/new_framework_defaults.rb
Rails.application.config.active_record.belongs_to_required_by_default = true

Also remove any reference to ActiveRecord::Base in your model files (or simply delete the files if apply). For example, the autogenerated app/models/application_record.rb file.

For Rails 3 and Rails 4:

Use -O(Capital 'O') or --skip-activerecord option to generate an application without a database.

rails new myApp -O

or

rails new myApp --skip-activerecord

This Answer is reshared from here

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For Rails 5:

Use --skip-active-record option to generate an application without a database

Notice the extra hyphen '-' as opposed to previous Rails versions.

rails new myApp --skip-active-record

Uncomment this line in the environment.rb file:

config.frameworks -= [ :active_record, :active_resource, :action_mailer]

In Rails 4 when starting a new project you can use -O or --skip-active-record

rails new my_project -O
rails new my_project --skip-active-record

If you've already created a project you will need to comment

 require "active_record/railtie"

from config/application.rb and

 config.active_record.migration_error = :page_load

from config/environments/development.rb