Backwards compatibility for ApplicationRecord.

If ApplicationRecord is not defined, then we should fall back to
ActiveRecord::Base without anything breaking.

Author: wangjohn

activerecord/lib/active_record/application_configuration.rb

@@ -4,7 +4,7 @@ module ApplicationConfiguration
 
     module ClassMethods
       def application_record(klass = nil)
-        return ActiveRecord::Base unless klass
+        return base_app_record unless klass
 
         klass = klass.class unless klass.respond_to?(:parents)
 
@@ -13,9 +13,15 @@ def application_record(klass = nil)
         elsif app_record = klass.parents.detect { |p| p.respond_to?(:application_record) }
           app_record
         else
-          ActiveRecord::Base
+          base_app_record
         end
       end
+
+      private
+
+        def base_app_record
+          @base_app_record ||= defined?(ApplicationRecord) ? ApplicationRecord : ActiveRecord::Base
+        end
     end
   end
 end

activerecord/lib/active_record/attribute_methods.rb

@@ -105,7 +105,7 @@ def instance_method_already_implemented?(method_name)
           raise DangerousAttributeError, "#{method_name} is defined by Active Record"
         end
 
-        if superclass == Base || superclass == ApplicationRecord
+        if superclass == ActiveRecord::Base.application_record || superclass == Base
           super
         else
           # If B < A and A defines its own attribute method, then we don't want to overwrite that.

activerecord/lib/active_record/reflection.rb

@@ -630,7 +630,7 @@ def source_reflection_name # :nodoc:
 Ambiguous source reflection for through association.  Please specify a :source
 directive on your declaration like:
 
-  class #{active_record.name} < ApplicationRecord
+  class #{active_record.name} < ActiveRecord::Base.application_record
     #{macro} :#{name}, #{example_options}
   end
 

activerecord/lib/active_record/schema_migration.rb

@@ -3,7 +3,7 @@
 require 'active_record/base'
 
 module ActiveRecord
-  class SchemaMigration < ApplicationRecord
+  class SchemaMigration < ActiveRecord::Base.application_record
     class << self
 
       def table_name

activerecord/lib/active_record/translation.rb

@@ -6,7 +6,7 @@ module Translation
     def lookup_ancestors #:nodoc:
       klass = self
       classes = [klass]
-      return classes if klass == ActiveRecord::Base || klass == ApplicationRecord
+      return classes if klass == ActiveRecord::Base.application_record || klass == ActiveRecord::Base
 
       while klass != klass.base_class
         classes << klass = klass.superclass

railties/test/application/loading_test.rb

@@ -87,11 +87,11 @@ class Post < ActiveRecord::Base
     require "#{rails_root}/config/environment"
     setup_ar!
 
-    assert_equal [ApplicationRecord, ActiveRecord::SchemaMigration], ActiveRecord::Base.descendants
+    assert_equal [ActiveRecord::SchemaMigration, ApplicationRecord], ActiveRecord::Base.descendants
     get "/load"
-    assert_equal [ApplicationRecord, Post], ActiveRecord::Base.descendants
+    assert_equal [ActiveRecord::SchemaMigration, Post, ApplicationRecord], ActiveRecord::Base.descendants
     get "/unload"
-    assert_equal [ApplicationRecord], ActiveRecord::Base.descendants
+    assert_equal [ActiveRecord::SchemaMigration, ApplicationRecord], ActiveRecord::Base.descendants
   end
 
   test "initialize cant be called twice" do