Reset current scopes in AR using an on_unload callback

Rails 6.0 and Rails 6.1 didn't support the undocumented `before_remove_const` in
`zeitwerk` mode. I noticed this cleanup in AR was not being executed, and
restored the original code for Rails 7.

However, invoking `respond_to?` in an `on_unload` callback may have unexpected
side-effects, as seen in #44125. So, this patch reimplements the cleanup in a
more modern way.

Fixes #44125.

activerecord/lib/active_record/railtie.rb

@@ -389,5 +389,15 @@ class Railtie < Rails::Railtie # :nodoc:
         end
       end
     end
+
+    initializer "active_record.unregister_current_scopes_on_unload" do |app|
+      config.after_initialize do
+        unless app.config.cache_classes
+          Rails.autoloaders.main.on_unload do |_cpath, value, _abspath|
+            value.current_scope = nil if value.is_a?(Class) && value < ActiveRecord::Base
+          end
+        end
+      end
+    end
   end
 end

activerecord/lib/active_record/scoping/default.rb

@@ -48,10 +48,6 @@ def scope_attributes? # :nodoc:
           super || default_scopes.any? || respond_to?(:default_scope)
         end
 
-        def before_remove_const # :nodoc:
-          self.current_scope = nil
-        end
-
         # Checks if the model has any default scopes. If all_queries
         # is set to true, the method will check if there are any
         # default_scopes for the model  where +all_queries+ is true.

activerecord/test/cases/base_test.rb

@@ -1300,20 +1300,6 @@ def test_clear_cache!
     assert_equal c1, c2
   end
 
-  def test_before_remove_const_resets_the_current_scope
-    # Done this way because a class cannot be defined in a method using the
-    # class keyword.
-    Object.const_set(:ReloadableModel, Class.new(ActiveRecord::Base))
-    ReloadableModel.current_scope = ReloadableModel.all
-    assert_not_nil ActiveRecord::Scoping::ScopeRegistry.current_scope(ReloadableModel) # precondition
-
-    ReloadableModel.before_remove_const
-
-    assert_nil ActiveRecord::Scoping::ScopeRegistry.current_scope(ReloadableModel)
-  ensure
-    Object.send(:remove_const, :ReloadableModel)
-  end
-
   def test_marshal_round_trip
     expected = posts(:welcome)
     marshalled = Marshal.dump(expected)

railties/lib/rails/application/finisher.rb

@@ -35,10 +35,6 @@ module Finisher
               ActiveSupport::Dependencies._autoloaded_tracked_classes << value
             end
           end
-
-          autoloader.on_unload do |_cpath, value, _abspath|
-            value.before_remove_const if value.respond_to?(:before_remove_const)
-          end
         end
 
         autoloader.setup

railties/test/application/initializers/frameworks_test.rb

@@ -338,5 +338,18 @@ def show
       require "#{app_path}/config/environment"
       assert_not_predicate ActiveRecord::Base.connection_pool, :active_connection?
     end
+
+    test "Current scopes in AR models are reset on reloading" do
+      rails %w(generate model post)
+      rails %w(db:migrate)
+      require "#{app_path}/config/environment"
+
+      Post.current_scope = Post
+      assert_not_nil ActiveRecord::Scoping::ScopeRegistry.current_scope(Post) # precondition
+
+      ActiveSupport::Dependencies.clear
+
+      assert_nil ActiveRecord::Scoping::ScopeRegistry.current_scope(Post)
+    end
   end
 end

railties/test/application/zeitwerk_integration_test.rb

@@ -262,34 +262,6 @@ def once_autoloader.reload
     assert_equal %i(main_autoloader), $zeitwerk_integration_reload_test
   end
 
-  test "reloading invokes before_remove_const" do
-    $before_remove_const_invoked = false
-
-    app_file "app/models/foo.rb", <<~RUBY
-      # While the most common use case is classes/modules, the contract does not
-      # require values to be so. Let's weaken the test down to Object.new.
-      Foo = Object.new
-      def Foo.before_remove_const
-        $before_remove_const_invoked = true
-      end
-    RUBY
-
-    app_file "app/models/bar.rb", <<~RUBY
-      # This object does not implement before_remove_const. We define it to make
-      # sure reloading does not raise. That is, it does not blindly invoke the
-      # hook on all unloaded objects.
-      Bar = Object.new
-    RUBY
-
-    boot
-
-    assert Foo
-    assert Bar
-    ActiveSupport::Dependencies.clear
-
-    assert $before_remove_const_invoked
-  end
-
   test "reloading clears autoloaded tracked classes" do
     eval <<~RUBY
       class Parent