[WIP] Use zeitwerk on load instead of hooking descendants subclasses

app/models/authenticator/base.rb

@@ -2,6 +2,11 @@ module Authenticator
   class Base
     include Vmdb::Logging
 
+    # Simulate being a STI table class for proper descendant class loading
+    def self.inheritance_column
+      "type"
+    end
+
     def self.validate_config(_config)
       []
     end

app/models/miq_request_workflow.rb

@@ -4,9 +4,6 @@ class MiqRequestWorkflow
   include Vmdb::Logging
   include DialogFieldValidation
 
-  # We rely on MiqRequestWorkflow's descendants to be comprehensive
-  singleton_class.send(:prepend, DescendantLoader::ArDescendantsWithLoader)
-
   attr_accessor :requester, :values, :last_vm_id
   attr_writer :dialogs
 

config/initializers/zeitwerk.rb

@@ -9,3 +9,27 @@
 Rails.autoloaders.main.collapse(Rails.root.join("lib/manageiq/reporting/charting"))
 Rails.autoloaders.main.collapse(Rails.root.join("lib/ansible/runner/credential"))
 Rails.autoloaders.main.collapse(Rails.root.join("lib/pdf_generator"))
+
+excluded_from_eager_load = %w[
+  ApplicationRecord
+  MiqDecorator
+  OrchestrationStack::Status
+  ResourcePool
+]
+
+DescendantLoader.instance.discovered_parent_child_classes.each do |parent, children|
+  next if excluded_from_eager_load.include?(parent)
+
+  puts "Registering on_load for class: #{parent}"
+  Rails.autoloaders.main.on_load(parent.to_s) do |klass, _abspath|
+    puts "Running on_load for class: #{klass} with children: #{children}"
+    children.each do |child|
+      begin
+        child.safe_constantize
+      rescue NameError => err
+        puts "!!! FAILED to load #{child} for parent: #{klass} with inheritance column: #{klass.try(:inheritance_column)} reason: #{err}. Error class: #{err.class.name}"
+        raise
+      end
+    end
+  end
+end

lib/acts_as_ar_model.rb

@@ -26,6 +26,11 @@ def self.includes_to_references(_inc)
     []
   end
 
+  # Simulate being a STI table class for proper descendant class loading
+  def self.inheritance_column
+    "type"
+  end
+
   class << self; alias_method :base_model, :base_class; end
 
   #

lib/extensions/descendant_loader.rb

@@ -242,6 +242,10 @@ def clear_class_inheritance_relationships
   include Cache
   include Mapper
 
+  def discovered_parent_child_classes
+    @discovered_parent_child_classes ||= class_inheritance_relationships.select { |_k, names| names.length > 0 }
+  end
+
   def load_subclasses(parent)
     names_to_load = class_inheritance_relationships[parent.to_s].dup
     while (name = names_to_load.shift)
@@ -261,29 +265,6 @@ def scoped_name(name, scopes)
     end
   end
 
-  module ArDescendantsWithLoader
-    def descendants
-      if Vmdb::Application.instance.initialized? && !defined? @loaded_descendants
-        @loaded_descendants = true
-        DescendantLoader.instance.load_subclasses(self)
-      end
-
-      super
-    end
-
-    # Rails 6.1 added an alias for subclasss to call direct_descendants in:
-    # https://github.com/rails/rails/commit/8f8aa857e084b76b1120edaa9bb9ce03ba1e6a19
-    # We need to get in front of it, like we do for descendants.
-    def subclasses
-      if Vmdb::Application.instance.initialized? && !defined? @loaded_descendants
-        @loaded_descendants = true
-        DescendantLoader.instance.load_subclasses(self)
-      end
-
-      super
-    end
-  end
-
   module AsDependenciesClearWithLoader
     def clear
       DescendantLoader.instance.clear_class_inheritance_relationships
@@ -292,12 +273,6 @@ def clear
   end
 end
 
-# Patch Class to support non-AR models in the models directory
-Class.prepend(DescendantLoader::ArDescendantsWithLoader)
-# Patch ActiveRecord specifically to get ahead of ActiveSupport::DescendantsTracker
-#   The patching of Class does not put it in the right place in the ancestors chain
-ActiveRecord::Base.singleton_class.prepend(DescendantLoader::ArDescendantsWithLoader)
-
 at_exit do
   DescendantLoader.instance.save_cache!
 end