How to add active record database back into a Rails 5 created with --skip-active-record

This is what a I did:

1. Go one level up of your folder app and run the rails new command again, hit enter every time to override the conflicts

rails new my_app_name -d mysql

2. Use the git tool to see changes in your app files (RubyMine, gitg, smartgit)

Files changed:

config/application.rb
config/environments/development.rb
config/environments/production.rb
config/initializers/wrap_parameters.rb
Gemfile
bin/setup
bin/update
test/test_helper.rb

New files

config/database.yml
app/models/application_record.rb
db/seeds.rb

Details(diff view - for removed + added):

config/application.rb

-# 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 "action_cable/engine"
-require "sprockets/railtie"
-require "rails/test_unit/railtie"
+require 'rails/all'

config/environments/development.rb

+# Raise an error on page load if there are pending migrations.
+config.active_record.migration_error = :page_load

config/environments/production.rb

+# Do not dump schema after migrations. +config.active_record.dump_schema_after_migration = false

Gemfile

+# Use mysql as the database for Active Record
+gem 'mysql2', '>= 0.3.18', '< 0.6.0'

bin/setup

+# puts "\n== Copying sample files =="
+# unless File.exist?('config/database.yml')
+#   cp 'config/database.yml.sample', 'config/database.yml'
+# end

+puts "\n== Preparing database =="
+system! 'bin/rails db:setup'

bin/update

+puts "\n== Updating database =="
+system! 'bin/rails db:migrate'

app/models/application_record.rb

+class ApplicationRecord < ActiveRecord::Base
+  self.abstract_class = true
+end

config/database.yml

+default: &default
+  adapter: mysql2
+  encoding: utf8
+  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
+  username: root
+  password:
+  host: localhost

+development:
+  <<: *default
+  database: app-without-activerecord_development

+test:
+  <<: *default
+  database: app-without-activerecord_test

+production:
+  <<: *default
+  database: app-without-activerecord_production
+  username: app-without-activerecord
+  password: <%= ENV['APP-WITHOUT-ACTIVERECORD_DATABASE_PASSWORD'] %>

the first thing that I did after reading your question was to create 2 new projects on my computer

rails new test-db and rails new test-no-db --skip-active-record now I should have 2 projects that are identical but 1 does not have active-record now can can run a difference between the 2 directories diff --brief -r test-db/ test-no-db/

files that change

1) Gemfile

# Use postgresql as the database for Active Record
gem 'pg', '>= 0.18', '< 2.0'

2) app/assets/javascripts/application.js

//= require activestorage

3) Add this file

#test-db/app/models/application_record.rb
class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end

4) test-db/bin/setup

chdir APP_ROOT do
  ...
  puts "\n== Preparing database =="
  system! 'bin/rails db:setup'
  ...
end

you can go threw the rest of the files to see what is different. I know that this is not the solution that you wanted but its what I would have done