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.

1 files changed, 62 insertions, 0 deletions

diff --git a/railties/test/commands/db_system_change_test.rb b/railties/test/commands/db_system_change_test.rb
new file mode 100644
index 0000000000..bac757f1eb
--- /dev/null
+++ b/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