Make automatically synchronize test schema work inside engine

In Rails engine, migration files are in under `db/migrate` of engine.
Therefore, when rake task is executed in engine, `db/migrate` is
automatically added to `DatabaseTasks.migrations_paths`.
https://github.com/rails/rails/blob/a18cf23a9cbcbeed61e8049442640c7153e0a8fb/activerecord/lib/active_record/railtie.rb#L39..L43

However, if execute the rake task under dummy app, migration files will not
be loaded because engine's migration path setting process is not called.

Therefore, in order to load migration files correctly, it is necessary to
execute rake task under engine.

Fixes #30765

2 files changed, 34 insertions, 0 deletions

diff --git a/railties/lib/rails/tasks/engine.rake b/railties/lib/rails/tasks/engine.rake
index 3ce49a1641..c967c54aa5 100644
--- a/railties/lib/rails/tasks/engine.rake
+++ b/railties/lib/rails/tasks/engine.rake
@@ -70,6 +70,9 @@ namespace :db do
 
   desc "Retrieves the current schema version number"
   app_task "version"
+
+  # desc 'Load the test schema'
+  app_task "test:prepare"
 end
 
 def find_engine_path(path)
diff --git a/railties/test/engine/test_test.rb b/railties/test/engine/test_test.rb
new file mode 100644
index 0000000000..18af85a0aa
--- /dev/null
+++ b/railties/test/engine/test_test.rb
@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require "abstract_unit"
+
+class Rails::Engine::TestTest < ActiveSupport::TestCase
+  setup do
+    @destination_root = Dir.mktmpdir("bukkits")
+    Dir.chdir(@destination_root) { `bundle exec rails plugin new bukkits --mountable` }
+  end
+
+  teardown do
+    FileUtils.rm_rf(@destination_root)
+  end
+
+  test "automatically synchronize test schema" do
+    Dir.chdir(plugin_path) do
+      # In order to confirm that migration files are loaded, generate multiple migration files.
+      `bin/rails generate model user name:string;
+       bin/rails generate model todo name:string;
+       RAILS_ENV=development bin/rails db:migrate`
+
+      output = `bin/rails test test/models/bukkits/user_test.rb`
+      assert_includes(output, "0 runs, 0 assertions, 0 failures, 0 errors, 0 skips")
+    end
+  end
+
+  private
+    def plugin_path
+      "#{@destination_root}/bukkits"
+    end
+end