Introduce ApplicationRecord, an Active Record layer supertype

It's pretty common for folks to monkey patch `ActiveRecord::Base` to
work around an issue or introduce extra functionality. Instead of
shoving even more stuff in `ActiveRecord::Base`, `ApplicationRecord` can
hold all those custom work the apps may need.

Now, we don't wanna encourage all of the application models to inherit
from `ActiveRecord::Base`, but we can encourage all the models that do,
to inherit from `ApplicationRecord`.

Newly generated applications have `app/models/application_record.rb`
present by default. The model generators are smart enough to recognize
that newly generated models have to inherit from `ApplicationRecord`,
but only if it's present.

1 files changed, 8 insertions, 1 deletions

diff --git a/activerecord/lib/rails/generators/active_record/model/model_generator.rb b/activerecord/lib/rails/generators/active_record/model/model_generator.rb
index 395951ac9d..e4c9539362 100644
--- a/activerecord/lib/rails/generators/active_record/model/model_generator.rb
+++ b/activerecord/lib/rails/generators/active_record/model/model_generator.rb
@@ -43,9 +43,16 @@ module ActiveRecord
 
         # Used by the migration template to determine the parent name of the model
         def parent_class_name
-          options[:parent] || "ActiveRecord::Base"
+          options[:parent] || determine_default_parent_class
         end
 
+        def determine_default_parent_class
+          if File.exist?('app/models/application_record.rb')
+            "ApplicationRecord"
+          else
+            "ActiveRecord::Base"
+          end
+        end
     end
   end
 end