Present the bug

Gemfile

@@ -18,6 +18,8 @@ gem "puma", "~> 5.0"
 # Windows does not include zoneinfo files, so bundle the tzinfo-data gem
 gem "tzinfo-data", platforms: %i[ mingw mswin x64_mingw jruby ]
 
+gem "async"
+
 group :development, :test do
   # See https://guides.rubyonrails.org/debugging_rails_applications.html#debugging-with-the-debug-gem
   gem "debug", platforms: %i[ mri mingw x64_mingw ]
@@ -27,4 +29,3 @@ group :development do
   # Speed up commands on slow machines / big apps [https://github.com/rails/spring]
   # gem "spring"
 end
-

Gemfile.lock

@@ -66,19 +66,27 @@ GEM
       i18n (>= 1.6, < 2)
       minitest (>= 5.1)
       tzinfo (~> 2.0)
+    async (2.0.1)
+      console (~> 1.10)
+      io-event (~> 1.0.0)
+      timers (~> 4.1)
     builder (3.2.4)
     concurrent-ruby (1.1.9)
+    console (1.14.0)
+      fiber-local
     crass (1.0.6)
     debug (1.4.0)
       irb (>= 1.3.6)
       reline (>= 0.2.7)
     digest (3.1.0)
     erubi (1.10.0)
+    fiber-local (1.0.0)
     globalid (1.0.0)
       activesupport (>= 5.0)
     i18n (1.10.0)
       concurrent-ruby (~> 1.0)
     io-console (0.5.11)
+    io-event (1.0.5)
     io-wait (0.2.1)
     irb (1.4.1)
       reline (>= 0.3.0)
@@ -155,6 +163,7 @@ GEM
     strscan (3.0.1)
     thor (1.2.1)
     timeout (0.2.0)
+    timers (4.3.3)
     tzinfo (2.0.4)
       concurrent-ruby (~> 1.0)
     websocket-driver (0.7.5)
@@ -166,6 +175,7 @@ PLATFORMS
   x86_64-linux
 
 DEPENDENCIES
+  async
   debug
   puma (~> 5.0)
   rails (~> 7.0.2, >= 7.0.2.3)

README.md

@@ -1,24 +1,23 @@
 # README
 
-This README would normally document whatever steps are necessary to get the
-application up and running.
+Run `rails t test/models/my_model_test.rb`
 
-Things you may want to cover:
+The output is:
 
-* Ruby version
+```
+ r t test/models/my_model_test.rb
+Running 1 tests in a single process (parallelization threshold is 50)
+Run options: --seed 2478
 
-* System dependencies
+# Running:
 
-* Configuration
+uninitialized constant MyModelTest::MyModel
 
-* Database creation
+          MyModel.run_something
+          ^^^^^^^
+uninitialized constant MyModelTest::MyModel
 
-* Database initialization
-
-* How to run the test suite
-
-* Services (job queues, cache servers, search engines, etc.)
-
-* Deployment instructions
-
-* ...
+          MyModel.run_something
+          ^^^^^^^
+something
+```

app/models/my_model.rb

@@ -0,0 +1,5 @@
+class MyModel
+  def self.run_something
+    puts "something"
+  end
+end

config/environments/test.rb

@@ -14,7 +14,7 @@
   # Eager loading loads your whole application. When running a single test locally,
   # this probably isn't necessary. It's a good idea to do in a continuous integration
   # system, or in some way before deploying your code.
-  config.eager_load = ENV["CI"].present?
+  config.eager_load = false # Setting to true fixes the test failure
 
   # Configure public file server for tests with Cache-Control for performance.
   config.public_file_server.enabled = true

test/models/my_model_test.rb

@@ -0,0 +1,15 @@
+require "test_helper"
+
+class MyModelTest < ActiveSupport::TestCase
+  test "async run" do
+    Async do |task|
+      3.times do |i|
+        task.async do
+          MyModel.run_something
+        rescue => e
+          puts e
+        end
+      end
+    end.wait
+  end
+end