Changing the guides: explanation for ApplicationRecord.

Author: wangjohn

guides/source/active_record_basics.md

@@ -159,6 +159,15 @@ p.name = "Some Book"
 puts p.name # "Some Book"
 ```
 
+Once you subclass `ApplicationRecord`, you get all of the functionality that is
+defined in the `ActiveRecord::Base` class since `ApplicationRecord` is itself a
+subclass of `ActiveRecord::Base`. The `ApplicationRecord` class allows you to
+configure Active Record without stepping over other applications that you would
+possibly be using, since every application you create has its own
+`ApplicationRecord`. Configurations set in `ActiveRecord::Base` are global to all
+applications using Active Record, but configurations set on `ApplicationRecord`
+are local to that application.
+
 Overriding the Naming Conventions
 ---------------------------------
 

guides/source/getting_started.md

@@ -766,7 +766,10 @@ There isn't much to this file - but note that the `Post` class inherits from
 `ApplicationRecord`. Active Record supplies a great deal of functionality to
 your Rails models for free, including basic database CRUD (Create, Read, Update,
 Destroy) operations, data validation, as well as sophisticated search support
-and the ability to relate multiple models to one another.
+and the ability to relate multiple models to one another. All you need to do
+to get all of this functionality is to subclass `ApplicationRecord` (which is
+itself a subclass of `ActiveRecord::Base` where all of the functionality is
+defined).
 
 Rails includes methods to help you validate the data that you send to models.
 Open the `app/models/post.rb` file and edit it: