Add rails db:system:change command

Add `rails db:system:change` command for changing databases.

```
bin/rails db:system:change --to=postgresql
   force  config/database.yml
    gsub  Gemfile
```

The change command copies a template `config/database.yml` with
the target database adapter into your app, and replaces your database
gem with the target database gem.

railties/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   Add `rails db:system:change` command for changing databases.
+
+    ```
+    bin/rails db:system:change --to=postgresql
+       force  config/database.yml
+        gsub  Gemfile
+    ```
+
+    The change command copies a template `config/database.yml` with the target database adapter into your app, and replaces your database gem with the target database gem.
+
+    *Gannon McGibbon*
+
 *   Use original `bundler` environment variables during the process of generating a new rails project.
 
     *Marco Costa*

railties/lib/rails/commands/db/system/change/change_command.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+require "rails/generators"
+require "rails/generators/rails/db/system/change/change_generator"
+
+module Rails
+  module Command
+    module Db
+      module System
+        class ChangeCommand < Base # :nodoc:
+          class_option :to, desc: "The database system to switch to."
+
+          def perform
+            Rails::Generators::Db::System::ChangeGenerator.start
+          end
+        end
+      end
+    end
+  end
+end

railties/lib/rails/generators.rb

@@ -220,6 +220,7 @@ module Rails
         rails.delete("encryption_key_file")
         rails.delete("master_key")
         rails.delete("credentials")
+        rails.delete("db:system:change")
 
         hidden_namespaces.each { |n| groups.delete(n.to_s) }
 

railties/lib/rails/generators/rails/db/system/change/change_generator.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+require "rails/generators/base"
+
+module Rails
+  module Generators
+    module Db
+      module System
+        class ChangeGenerator < Base # :nodoc:
+          include Database
+          include AppName
+
+          class_option :to, required: true,
+            desc: "The database system to switch to."
+
+          def self.default_generator_root
+            path = File.expand_path(File.join(base_name, "app"), base_root)
+            path if File.exist?(path)
+          end
+
+          def initialize(*)
+            super
+
+            unless DATABASES.include?(options[:to])
+              raise Error, "Invalid value for --to option. Supported for preconfiguration are: #{DATABASES.join(", ")}."
+            end
+
+            opt = options.dup
+            opt[:database] ||= opt[:to]
+            self.options = opt.freeze
+          end
+
+          def edit_database_config
+            template("config/databases/#{options[:database]}.yml", "config/database.yml")
+          end
+
+          def edit_gemfile
+            database_gem_name, _ = gem_for_database
+            gsub_file("Gemfile", all_database_gems_regex, database_gem_name)
+          end
+
+          private
+            def all_database_gems
+              DATABASES.map { |database| gem_for_database(database) }
+            end
+
+            def all_database_gems_regex
+              all_database_gem_names = all_database_gems.map(&:first)
+              /(\b#{all_database_gem_names.join('\b|\b')}\b)/
+            end
+        end
+      end
+    end
+  end
+end

railties/test/commands/db_system_change_test.rb

@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+require "isolation/abstract_unit"
+require "rails/command"
+require "rails/commands/db/system/change/change_command"
+
+class Rails::Command::Db::System::ChangeCommandTest < ActiveSupport::TestCase
+  include ActiveSupport::Testing::Isolation
+
+  setup { build_app }
+
+  teardown { teardown_app }
+
+  test "change to existing database" do
+    change_database(to: "sqlite3")
+
+    output = change_database(to: "sqlite3")
+
+    assert_match "identical  config/database.yml", output
+    assert_match "gsub  Gemfile", output
+  end
+
+  test "change to invalid database" do
+    output = change_database(to: "invalid-db")
+
+    assert_match <<~MSG.squish, output
+      Invalid value for --to option.
+      Supported for preconfiguration are:
+      mysql, postgresql, sqlite3, oracle, frontbase,
+      ibm_db, sqlserver, jdbcmysql, jdbcsqlite3,
+      jdbcpostgresql, jdbc.
+    MSG
+  end
+
+  test "change to postgresql" do
+    output = change_database(to: "postgresql")
+
+    assert_match "force  config/database.yml", output
+    assert_match "gsub  Gemfile", output
+  end
+
+  test "change to mysql" do
+    output = change_database(to: "mysql")
+
+    assert_match "force  config/database.yml", output
+    assert_match "gsub  Gemfile", output
+  end
+
+  test "change to sqlite3" do
+    change_database(to: "postgresql")
+    output = change_database(to: "sqlite3")
+
+    assert_match "force  config/database.yml", output
+    assert_match "gsub  Gemfile", output
+  end
+
+  private
+    def change_database(to:, **options)
+      args = ["--to", to]
+      rails "db:system:change", args, **options
+    end
+end

railties/test/generators/db_system_change_generator_test.rb

@@ -0,0 +1,78 @@
+# frozen_string_literal: true
+
+require "generators/generators_test_helper"
+require "rails/generators/rails/db/system/change/change_generator"
+
+module Rails
+  module Generators
+    module Db
+      module System
+        class ChangeGeneratorTest < Rails::Generators::TestCase
+          include GeneratorsTestHelper
+
+          setup do
+            copy_gemfile(
+              GemfileEntry.new("sqlite3", nil, "Use sqlite3 as the database for Active Record")
+            )
+          end
+
+          test "change to invalid database" do
+            output = capture(:stderr) do
+              run_generator ["--to", "invalid-db"]
+            end
+
+            assert_match <<~MSG.squish, output
+              Invalid value for --to option.
+              Supported for preconfiguration are:
+              mysql, postgresql, sqlite3, oracle, frontbase,
+              ibm_db, sqlserver, jdbcmysql, jdbcsqlite3,
+              jdbcpostgresql, jdbc.
+            MSG
+          end
+
+          test "change to postgresql" do
+            run_generator ["--to", "postgresql"]
+
+            assert_file("config/database.yml") do |content|
+              assert_match "adapter: postgresql", content
+              assert_match "database: test_app", content
+            end
+
+            assert_file("Gemfile") do |content|
+              assert_match "# Use pg as the database for Active Record", content
+              assert_match "gem 'pg'", content
+            end
+          end
+
+          test "change to mysql" do
+            run_generator ["--to", "mysql"]
+
+            assert_file("config/database.yml") do |content|
+              assert_match "adapter: mysql2", content
+              assert_match "database: test_app", content
+            end
+
+            assert_file("Gemfile") do |content|
+              assert_match "# Use mysql2 as the database for Active Record", content
+              assert_match "gem 'mysql2'", content
+            end
+          end
+
+          test "change to sqlite3" do
+            run_generator ["--to", "sqlite3"]
+
+            assert_file("config/database.yml") do |content|
+              assert_match "adapter: sqlite3", content
+              assert_match "db/development.sqlite3", content
+            end
+
+            assert_file("Gemfile") do |content|
+              assert_match "# Use sqlite3 as the database for Active Record", content
+              assert_match "gem 'sqlite3'", content
+            end
+          end
+        end
+      end
+    end
+  end
+end

railties/test/generators/generators_test_helper.rb

@@ -30,6 +30,12 @@ module GeneratorsTestHelper
   include ActiveSupport::Testing::Stream
   include ActiveSupport::Testing::MethodCallAssertions
 
+  GemfileEntry = Struct.new(:name, :version, :comment, :options, :commented_out) do
+    def initialize(name, version, comment, options = {}, commented_out = false)
+      super
+    end
+  end
+
   def self.included(base)
     base.class_eval do
       destination File.join(Rails.root, "tmp")
@@ -63,4 +69,34 @@ module GeneratorsTestHelper
     FileUtils.mkdir_p(destination)
     FileUtils.cp routes, File.join(destination, "routes.rb")
   end
+
+  def copy_gemfile(*gemfile_entries)
+    locals = gemfile_locals.merge(gemfile_entries: gemfile_entries)
+    gemfile = File.expand_path("../../lib/rails/generators/rails/app/templates/Gemfile.tt", __dir__)
+    gemfile = evaluate_template(gemfile, locals)
+    destination = File.join(destination_root)
+    File.write File.join(destination, "Gemfile"), gemfile
+  end
+
+  def evaluate_template(file, locals = {})
+    erb = ERB.new(File.read(file), nil, "-", "@output_buffer")
+    context = Class.new do
+      locals.each do |local, value|
+        class_attribute local, default: value
+      end
+    end
+    erb.result(context.new.instance_eval("binding"))
+  end
+
+  private
+    def gemfile_locals
+      {
+        skip_active_storage: true,
+        depend_on_bootsnap: false,
+        depend_on_listen: false,
+        spring_install: false,
+        depends_on_system_test: false,
+        options: ActiveSupport::OrderedOptions.new,
+      }
+    end
 end

railties/test/isolation/abstract_unit.rb

@@ -476,7 +476,7 @@ Module.new do
     `yarn build`
   end
 
-  `#{Gem.ruby} #{RAILS_FRAMEWORK_ROOT}/railties/exe/rails new #{app_template_path} --skip-gemfile --skip-listen --no-rc`
+  `#{Gem.ruby} #{RAILS_FRAMEWORK_ROOT}/railties/exe/rails new #{app_template_path} --skip-bundle --skip-listen --no-rc`
   File.open("#{app_template_path}/config/boot.rb", "w") do |f|
     f.puts "require 'rails/all'"
   end