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.

3 files changed, 76 insertions, 0 deletions

diff --git a/railties/lib/rails/commands/db/system/change/change_command.rb b/railties/lib/rails/commands/db/system/change/change_command.rb
new file mode 100644
index 0000000000..760c229c07
--- /dev/null
+++ b/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
diff --git a/railties/lib/rails/generators.rb b/railties/lib/rails/generators.rb
index 1e996a25ab..b835b3f3fd 100644
--- a/railties/lib/rails/generators.rb
+++ b/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) }
 
diff --git a/railties/lib/rails/generators/rails/db/system/change/change_generator.rb b/railties/lib/rails/generators/rails/db/system/change/change_generator.rb
new file mode 100644
index 0000000000..e7696dec0c
--- /dev/null
+++ b/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