Make `application_record_class` a module instance variable

Followup: https://github.com/rails/rails/pull/42442

activerecord/lib/active_record.rb

@@ -194,6 +194,9 @@ module ActiveRecord
   singleton_class.attr_accessor :warn_on_records_fetched_greater_than
   self.warn_on_records_fetched_greater_than = false
 
+  singleton_class.attr_accessor :application_record_class
+  self.application_record_class = nil
+
   def self.eager_load!
     super
     ActiveRecord::Locking.eager_load!

activerecord/lib/active_record/core.rb

@@ -146,8 +146,6 @@ module ActiveRecord
 
       class_attribute :default_shard, instance_writer: false
 
-      mattr_accessor :application_record_class, instance_accessor: false, default: nil
-
       # Sets the async_query_executor for an application. By default the thread pool executor
       # set to +nil+ which will not run queries in the background. Applications must configure
       # a thread pool executor to use this feature. Options are:
@@ -185,8 +183,8 @@ module ActiveRecord
       end
 
       def self.application_record_class? # :nodoc:
-        if Base.application_record_class
-          self == Base.application_record_class
+        if ActiveRecord.application_record_class
+          self == ActiveRecord.application_record_class
         else
           if defined?(ApplicationRecord) && self == ApplicationRecord
             true

activerecord/lib/active_record/inheritance.rb

@@ -163,12 +163,12 @@ module ActiveRecord
       # will share a database connection with Active Record. It is the class
       # that connects to your primary database.
       def primary_abstract_class
-        if Base.application_record_class && Base.application_record_class.name != name
-          raise ArgumentError, "The `primary_abstract_class` is already set to #{Base.application_record_class.inspect}. There can only be one `primary_abstract_class` in an application."
+        if ActiveRecord.application_record_class && ActiveRecord.application_record_class.name != name
+          raise ArgumentError, "The `primary_abstract_class` is already set to #{ActiveRecord.application_record_class.inspect}. There can only be one `primary_abstract_class` in an application."
         end
 
         self.abstract_class = true
-        Base.application_record_class = self
+        ActiveRecord.application_record_class = self
       end
 
       # Returns the value to be stored in the inheritance column for STI.

activerecord/test/cases/connection_adapters/connection_swapping_nested_test.rb

@@ -448,7 +448,7 @@ module ActiveRecord
             end
           end
         ensure
-          ActiveRecord::Base.application_record_class = nil
+          ActiveRecord.application_record_class = nil
           Object.send(:remove_const, :ApplicationRecord)
           ActiveRecord::Base.establish_connection :arunit
         end

activerecord/test/cases/primary_class_test.rb

@@ -27,7 +27,7 @@ class PrimaryClassTest < ActiveRecord::TestCase
     assert_predicate ApplicationRecord, :application_record_class?
     assert_predicate ApplicationRecord, :abstract_class?
   ensure
-    ActiveRecord::Base.application_record_class = nil
+    ActiveRecord.application_record_class = nil
     Object.send(:remove_const, :ApplicationRecord)
   end
 
@@ -46,7 +46,7 @@ class PrimaryClassTest < ActiveRecord::TestCase
     assert_not_predicate ActiveRecord::Base, :application_record_class?
     assert_not_predicate ActiveRecord::Base, :abstract_class?
   ensure
-    ActiveRecord::Base.application_record_class = nil
+    ActiveRecord.application_record_class = nil
   end
 
   def test_primary_abstract_class_cannot_be_reset
@@ -56,7 +56,7 @@ class PrimaryClassTest < ActiveRecord::TestCase
       PrimaryClassTest::AnotherAppRecord.primary_abstract_class
     end
   ensure
-    ActiveRecord::Base.application_record_class = nil
+    ActiveRecord.application_record_class = nil
   end
 
   def test_primary_abstract_class_is_used_over_application_record_if_set
@@ -75,7 +75,7 @@ class PrimaryClassTest < ActiveRecord::TestCase
     assert_not_predicate ActiveRecord::Base, :application_record_class?
     assert_not_predicate ActiveRecord::Base, :abstract_class?
   ensure
-    ActiveRecord::Base.application_record_class = nil
+    ActiveRecord.application_record_class = nil
     Object.send(:remove_const, :ApplicationRecord)
   end
 
@@ -96,7 +96,7 @@ class PrimaryClassTest < ActiveRecord::TestCase
     assert_not_predicate ApplicationRecord, :application_record_class?
     assert_predicate ApplicationRecord, :abstract_class?
   ensure
-    ActiveRecord::Base.application_record_class = nil
+    ActiveRecord.application_record_class = nil
     Object.send(:remove_const, :ApplicationRecord)
   end
 
@@ -110,7 +110,7 @@ class PrimaryClassTest < ActiveRecord::TestCase
       assert_predicate ApplicationRecord, :application_record_class?
       assert_equal ActiveRecord::Base.connection, ApplicationRecord.connection
     ensure
-      ActiveRecord::Base.application_record_class = nil
+      ActiveRecord.application_record_class = nil
       Object.send(:remove_const, :ApplicationRecord)
       ActiveRecord::Base.establish_connection :arunit
     end
@@ -125,7 +125,7 @@ class PrimaryClassTest < ActiveRecord::TestCase
       assert_predicate PrimaryClassTest::PrimaryAppRecord, :abstract_class?
       assert_equal ActiveRecord::Base.connection, PrimaryClassTest::PrimaryAppRecord.connection
     ensure
-      ActiveRecord::Base.application_record_class = nil
+      ActiveRecord.application_record_class = nil
       ActiveRecord::Base.establish_connection :arunit
     end
   end