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, 5 insertions, 5 deletions

diff --git a/guides/source/engines.md b/guides/source/engines.md
index a50ef9a95f..8382bde4d3 100644
--- a/guides/source/engines.md
+++ b/guides/source/engines.md
@@ -508,7 +508,7 @@ Turning the model into this:
 
 ```ruby
 module Blorgh
-  class Article < ActiveRecord::Base
+  class Article < ApplicationRecord
     has_many :comments
   end
 end
@@ -1129,7 +1129,7 @@ end
 ```ruby
 # Blorgh/app/models/article.rb
 
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   has_many :comments
 end
 ```
@@ -1150,7 +1150,7 @@ end
 ```ruby
 # Blorgh/app/models/article.rb
 
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   has_many :comments
   def summary
     "#{title}"
@@ -1171,7 +1171,7 @@ classes at run time allowing you to significantly modularize your code.
 ```ruby
 # MyApp/app/models/blorgh/article.rb
 
-class Blorgh::Article < ActiveRecord::Base
+class Blorgh::Article < ApplicationRecord
   include Blorgh::Concerns::Models::Article
 
   def time_since_created
@@ -1187,7 +1187,7 @@ end
 ```ruby
 # Blorgh/app/models/article.rb
 
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   include Blorgh::Concerns::Models::Article
 end
 ```