Use association reflections for search calls.

This seems to be the more modern approach, and avoids the flaky method cache memoisation that has cropped up in Rails 6. My one frustration is that the tests were green with the old approach, but that's not the case in my own upgraded-to-Rails-6 apps.

lib/thinking_sphinx/active_record/association_proxy.rb

@@ -1,8 +1,6 @@
 # frozen_string_literal: true
 
 module ThinkingSphinx::ActiveRecord::AssociationProxy
-  extend ActiveSupport::Concern
-
   def search(query = nil, options = {})
     perform_search super(*normalise_search_arguments(query, options))
   end
@@ -12,6 +10,7 @@ def search_for_ids(query = nil, options = {})
   end
 
   private
+
   def normalise_search_arguments(query, options)
     query, options = nil, query if query.is_a?(Hash)
     options[:ignore_scopes] = true

lib/thinking_sphinx/active_record/base.rb

@@ -9,8 +9,23 @@ module ThinkingSphinx::ActiveRecord::Base
     after_update  ThinkingSphinx::ActiveRecord::Callbacks::UpdateCallbacks
     after_commit  ThinkingSphinx::ActiveRecord::Callbacks::DeltaCallbacks
 
-    ::ActiveRecord::Associations::CollectionProxy.send :include,
-      ThinkingSphinx::ActiveRecord::AssociationProxy
+    if ActiveRecord::VERSION::STRING.to_i >= 5
+      [
+        ::ActiveRecord::Reflection::HasManyReflection,
+        ::ActiveRecord::Reflection::HasAndBelongsToManyReflection
+      ].each do |reflection_class|
+        reflection_class.include DefaultReflectionAssociations
+      end
+    else
+      ::ActiveRecord::Associations::CollectionProxy.send :include,
+        ThinkingSphinx::ActiveRecord::AssociationProxy
+    end
+  end
+
+  module DefaultReflectionAssociations
+    def extensions
+      super + [ThinkingSphinx::ActiveRecord::AssociationProxy]
+    end
   end
 
   module ClassMethods