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.

2 files changed, 114 insertions, 0 deletions

diff --git a/railties/test/generators/db_system_change_generator_test.rb b/railties/test/generators/db_system_change_generator_test.rb
new file mode 100644
index 0000000000..11726db244
--- /dev/null
+++ b/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
diff --git a/railties/test/generators/generators_test_helper.rb b/railties/test/generators/generators_test_helper.rb
index 25d5dba1d8..975a204af4 100644
--- a/railties/test/generators/generators_test_helper.rb
+++ b/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