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, 7 deletions

diff --git a/guides/source/active_record_basics.md b/guides/source/active_record_basics.md
index abb22f9cb8..8bd135ddd5 100644
--- a/guides/source/active_record_basics.md
+++ b/guides/source/active_record_basics.md
@@ -132,10 +132,10 @@ Creating Active Record Models
 -----------------------------
 
 It is very easy to create Active Record models. All you have to do is to
-subclass the `ActiveRecord::Base` class and you're good to go:
+subclass the `ApplicationRecord` class and you're good to go:
 
 ```ruby
-class Product < ActiveRecord::Base
+class Product < ApplicationRecord
 end
 ```
 
@@ -168,11 +168,12 @@ What if you need to follow a different naming convention or need to use your
 Rails application with a legacy database? No problem, you can easily override
 the default conventions.
 
-You can use the `ActiveRecord::Base.table_name=` method to specify the table
-name that should be used:
+`ApplicationRecord` inherits from `ActionController::Base`, which defines a
+number of helpful methods. You can use the `ActiveRecord::Base.table_name=`
+method to specify the table name that should be used:
 
 ```ruby
-class Product < ActiveRecord::Base
+class Product < ApplicationRecord
   self.table_name = "my_products"
 end
 ```
@@ -193,7 +194,7 @@ It's also possible to override the column that should be used as the table's
 primary key using the `ActiveRecord::Base.primary_key=` method:
 
 ```ruby
-class Product < ActiveRecord::Base
+class Product < ApplicationRecord
   self.primary_key = "product_id"
 end
 ```
@@ -320,7 +321,7 @@ they raise the exception `ActiveRecord::RecordInvalid` if validation fails.
 A quick example to illustrate:
 
 ```ruby
-class User < ActiveRecord::Base
+class User < ApplicationRecord
   validates :name, presence: true
 end