Include ActiveRecord::TestFixtures in the fixtures RBI

[mokevnin]

Motivation

Rails mixes ActiveRecord::TestFixtures into ActiveSupport::TestCase from the :active_support_test_case load hook in rails/test_help.rb. That file is required from an app's test_helper.rb, so the hook never runs during RBI generation, the include never makes it into the gem RBI, and Sorbet doesn't see the class methods the module contributes through mixes_in_class_methods — most visibly fixtures:

class PostTest < ActiveSupport::TestCase
  fixtures :all # Method `fixtures` does not exist on `T.class_of(PostTest)` (7003)
end

Today every Rails app using fixtures shims this by hand (a sorbet/rbi/shims entry, or a local DSL compiler that emits nothing but the include). ActiveRecordFixtures already targets ActiveSupport::TestCase and already knows Rails is loaded, so it's the natural place to make the include explicit.

Implementation

One line in decorate — mod.create_include("ActiveRecord::TestFixtures") — inside the existing create_path.

The early return now sits above the select! that drops fixture sets whose model constant can't be resolved, so the two cases it used to conflate are separated: an app with no fixture files at all still generates no RBI, while an app that has fixture files but no matching models gets an RBI with just the include (it writes fixtures :all too).

Tests

spec/tapioca/dsl/compilers/active_record_fixtures_spec.rb updated for the extra line, plus a new case for fixtures with no associated model at all; the whole file passes. bin/typecheck and rubocop are clean, and manual/ is regenerated via bin/docs.

[KaanOzkan]

Thanks, this is a unique change but I think it's beneficial to have it in the compiler as you said.

[KaanOzkan]

I couldn't find that initializer, I think it's being included through this load hook https://github.com/rails/rails/blob/4130768a1b0d95da640ac792920e54988cf2d12f/railties/lib/rails/test_help.rb#L15-L24. Can we change the wording here?

[mokevnin]

You're right, there is no such initializer — thanks for digging. It's the :active_support_test_case load hook in rails/test_help.rb, which an app requires from test_helper.rb, so it never runs during RBI generation. Reworded here, in manual/ and in the PR description.