Fix STI support for subclasses

- Rails 7.0 deprecated the `direct_descendants` method in favor of `subclasses`
- Ruby 3.1 added native `subclasses` support to `Class`

Ref #179

Ref:
- Alias direct_descendants as subclasses: rails/rails@8f8aa85 rails/rails#39505
- Filter reloaded classes in Class#subclasses and Class#descendants core exts: rails/rails@1aa2463 rails/rails#46144

lib/chrono_model/time_machine.rb

@@ -21,12 +21,35 @@ module TimeMachine
       ChronoModel.history_models[table_name] = history
 
       class << self
-        alias_method :direct_descendants_with_history, :direct_descendants
-        def direct_descendants
-          direct_descendants_with_history.reject(&:history?)
+        if Rails.version >= '7.0'
+          alias_method :subclasses_with_history, :subclasses
+          def subclasses
+            subclasses_with_history.reject(&:history?)
+          end
+
+          # `direct_descendants` is deprecated method in 7.0 and has been
+          # removed in 7.1
+          if method_defined?(:direct_descendants)
+            alias_method :direct_descendants_with_history, :subclasses_with_history
+            alias_method :direct_descendants, :subclasses
+          end
+
+          # Ruby 3.1 has a native subclasses method and descendants is
+          # implemented with recursion of subclasses
+          if Class.method_defined?(:subclasses)
+            def descendants_with_history
+              subclasses_with_history.concat(subclasses.flat_map(&:descendants))
+            end
+          end
+        else
+          alias_method :descendants_with_history, :descendants
+
+          alias_method :direct_descendants_with_history, :direct_descendants
+          def direct_descendants
+            direct_descendants_with_history.reject(&:history?)
+          end
         end
 
-        alias_method :descendants_with_history, :descendants
         def descendants
           descendants_with_history.reject(&:history?)
         end

spec/chrono_model/time_machine/sti_spec.rb

@@ -18,21 +18,50 @@ class ::Element < ActiveRecord::Base
     include ChronoModel::TimeMachine
   end
 
-  class ::Publication < Element
+  class ::Publication < ::Element
+  end
+
+  class ::Magazine < ::Publication
   end
 
   describe '.descendants' do
-    subject { Element.descendants }
+    subject { Element.descendants.map(&:name) }
 
-    it { is_expected.to_not include(Element::History) }
-    it { is_expected.to     include(Publication) }
+    it { is_expected.to match_array %w[Publication Magazine] }
   end
 
   describe '.descendants_with_history' do
-    subject { Element.descendants_with_history }
+    subject { Element.descendants_with_history.map(&:name) }
+
+    it { is_expected.to match_array %w[Element::History Publication Magazine] }
+  end
+
+  if Element.respond_to?(:direct_descendants)
+    describe '.direct_descendants' do
+      subject { Element.direct_descendants.map(&:name) }
+
+      it { is_expected.to match_array %w[Publication] }
+    end
+
+    describe '.direct_descendants_with_history' do
+      subject { Element.direct_descendants_with_history.map(&:name) }
 
-    it { is_expected.to include(Element::History) }
-    it { is_expected.to include(Publication) }
+      it { is_expected.to match_array %w[Element::History Publication] }
+    end
+  end
+
+  if Element.respond_to?(:subclasses)
+    describe '.subclasses' do
+      subject { Element.subclasses.map(&:name) }
+
+      it { is_expected.to match_array %w[Publication] }
+    end
+
+    describe '.subclasses_with_history' do
+      subject { Element.subclasses_with_history.map(&:name) }
+
+      it { is_expected.to match_array %w[Element::History Publication] }
+    end
   end
 
   describe 'timeline' do