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.

7 files changed, 148 insertions, 132 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
 
diff --git a/guides/source/active_record_callbacks.md b/guides/source/active_record_callbacks.md
index b5ad3e9411..d95c6c0e78 100644
--- a/guides/source/active_record_callbacks.md
+++ b/guides/source/active_record_callbacks.md
@@ -31,7 +31,7 @@ Callbacks are methods that get called at certain moments of an object's life cyc
 In order to use the available callbacks, you need to register them. You can implement the callbacks as ordinary methods and use a macro-style class method to register them as callbacks:
 
 ```ruby
-class User < ActiveRecord::Base
+class User < ApplicationRecord
   validates :login, :email, presence: true
 
   before_validation :ensure_login_has_a_value
@@ -48,7 +48,7 @@ end
 The macro-style class methods can also receive a block. Consider using this style if the code inside your block is so short that it fits in a single line:
 
 ```ruby
-class User < ActiveRecord::Base
+class User < ApplicationRecord
   validates :login, :email, presence: true
 
   before_create do
@@ -60,7 +60,7 @@ end
 Callbacks can also be registered to only fire on certain life cycle events:
 
 ```ruby
-class User < ActiveRecord::Base
+class User < ApplicationRecord
   before_validation :normalize_name, on: :create
 
   # :on takes an array as well
@@ -126,7 +126,7 @@ The `after_find` callback will be called whenever Active Record loads a record f
 The `after_initialize` and `after_find` callbacks have no `before_*` counterparts, but they can be registered just like the other Active Record callbacks.
 
 ```ruby
-class User < ActiveRecord::Base
+class User < ApplicationRecord
   after_initialize do |user|
     puts "You have initialized an object!"
   end
@@ -151,7 +151,7 @@ You have initialized an object!
 The `after_touch` callback will be called whenever an Active Record object is touched.
 
 ```ruby
-class User < ActiveRecord::Base
+class User < ApplicationRecord
   after_touch do |user|
     puts "You have touched an object"
   end
@@ -168,14 +168,14 @@ You have touched an object
 It can be used along with `belongs_to`:
 
 ```ruby
-class Employee < ActiveRecord::Base
+class Employee < ApplicationRecord
   belongs_to :company, touch: true
   after_touch do
     puts 'An Employee was touched'
   end
 end
 
-class Company < ActiveRecord::Base
+class Company < ApplicationRecord
   has_many :employees
   after_touch :log_when_employees_or_company_touched
 
@@ -266,11 +266,11 @@ Relational Callbacks
 Callbacks work through model relationships, and can even be defined by them. Suppose an example where a user has many articles. A user's articles should be destroyed if the user is destroyed. Let's add an `after_destroy` callback to the `User` model by way of its relationship to the `Article` model:
 
 ```ruby
-class User < ActiveRecord::Base
+class User < ApplicationRecord
   has_many :articles, dependent: :destroy
 end
 
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   after_destroy :log_destroy_action
 
   def log_destroy_action
@@ -297,7 +297,7 @@ As with validations, we can also make the calling of a callback method condition
 You can associate the `:if` and `:unless` options with a symbol corresponding to the name of a predicate method that will get called right before the callback. When using the `:if` option, the callback won't be executed if the predicate method returns false; when using the `:unless` option, the callback won't be executed if the predicate method returns true. This is the most common option. Using this form of registration it is also possible to register several different predicates that should be called to check if the callback should be executed.
 
 ```ruby
-class Order < ActiveRecord::Base
+class Order < ApplicationRecord
   before_save :normalize_card_number, if: :paid_with_card?
 end
 ```
@@ -307,7 +307,7 @@ end
 You can also use a string that will be evaluated using `eval` and hence needs to contain valid Ruby code. You should use this option only when the string represents a really short condition:
 
 ```ruby
-class Order < ActiveRecord::Base
+class Order < ApplicationRecord
   before_save :normalize_card_number, if: "paid_with_card?"
 end
 ```
@@ -317,7 +317,7 @@ end
 Finally, it is possible to associate `:if` and `:unless` with a `Proc` object. This option is best suited when writing short validation methods, usually one-liners:
 
 ```ruby
-class Order < ActiveRecord::Base
+class Order < ApplicationRecord
   before_save :normalize_card_number,
     if: Proc.new { |order| order.paid_with_card? }
 end
@@ -328,7 +328,7 @@ end
 When writing conditional callbacks, it is possible to mix both `:if` and `:unless` in the same callback declaration:
 
 ```ruby
-class Comment < ActiveRecord::Base
+class Comment < ApplicationRecord
   after_create :send_email_to_author, if: :author_wants_emails?,
     unless: Proc.new { |comment| comment.article.ignore_comments? }
 end
@@ -354,7 +354,7 @@ end
 When declared inside a class, as above, the callback methods will receive the model object as a parameter. We can now use the callback class in the model:
 
 ```ruby
-class PictureFile < ActiveRecord::Base
+class PictureFile < ApplicationRecord
   after_destroy PictureFileCallbacks.new
 end
 ```
@@ -374,7 +374,7 @@ end
 If the callback method is declared this way, it won't be necessary to instantiate a `PictureFileCallbacks` object.
 
 ```ruby
-class PictureFile < ActiveRecord::Base
+class PictureFile < ApplicationRecord
   after_destroy PictureFileCallbacks
 end
 ```
@@ -398,7 +398,7 @@ end
 By using the `after_commit` callback we can account for this case.
 
 ```ruby
-class PictureFile < ActiveRecord::Base
+class PictureFile < ApplicationRecord
   after_commit :delete_picture_file_from_disk, on: [:destroy]
 
   def delete_picture_file_from_disk
@@ -420,7 +420,7 @@ common, there are aliases for those operations:
 * `after_destroy_commit`
 
 ```ruby
-class PictureFile < ActiveRecord::Base
+class PictureFile < ApplicationRecord
   after_destroy_commit :delete_picture_file_from_disk
 
   def delete_picture_file_from_disk
diff --git a/guides/source/active_record_postgresql.md b/guides/source/active_record_postgresql.md
index 742db7be32..b592209d4b 100644
--- a/guides/source/active_record_postgresql.md
+++ b/guides/source/active_record_postgresql.md
@@ -39,7 +39,7 @@ create_table :documents do |t|
 end
 
 # app/models/document.rb
-class Document < ActiveRecord::Base
+class Document < ApplicationRecord
 end
 
 # Usage
@@ -63,7 +63,7 @@ add_index :books, :tags, using: 'gin'
 add_index :books, :ratings, using: 'gin'
 
 # app/models/book.rb
-class Book < ActiveRecord::Base
+class Book < ApplicationRecord
 end
 
 # Usage
@@ -97,7 +97,7 @@ ActiveRecord::Schema.define do
 end
 
 # app/models/profile.rb
-class Profile < ActiveRecord::Base
+class Profile < ApplicationRecord
 end
 
 # Usage
@@ -122,7 +122,7 @@ create_table :events do |t|
 end
 
 # app/models/event.rb
-class Event < ActiveRecord::Base
+class Event < ApplicationRecord
 end
 
 # Usage
@@ -150,7 +150,7 @@ create_table :events do |t|
 end
 
 # app/models/event.rb
-class Event < ActiveRecord::Base
+class Event < ApplicationRecord
 end
 
 # Usage
@@ -200,7 +200,7 @@ create_table :contacts do |t|
 end
 
 # app/models/contact.rb
-class Contact < ActiveRecord::Base
+class Contact < ApplicationRecord
 end
 
 # Usage
@@ -239,7 +239,7 @@ def down
 end
 
 # app/models/article.rb
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
 end
 
 # Usage
@@ -294,7 +294,7 @@ create_table :revisions do |t|
 end
 
 # app/models/revision.rb
-class Revision < ActiveRecord::Base
+class Revision < ApplicationRecord
 end
 
 # Usage
@@ -317,12 +317,12 @@ create_table :comments, id: :uuid, default: 'gen_random_uuid()' do |t|
 end
 
 # app/models/post.rb
-class Post < ActiveRecord::Base
+class Post < ApplicationRecord
   has_many :comments
 end
 
 # app/models/comment.rb
-class Comment < ActiveRecord::Base
+class Comment < ApplicationRecord
   belongs_to :post
 end
 ```
@@ -341,7 +341,7 @@ create_table :users, force: true do |t|
 end
 
 # app/models/device.rb
-class User < ActiveRecord::Base
+class User < ApplicationRecord
 end
 
 # Usage
@@ -370,7 +370,7 @@ create_table(:devices, force: true) do |t|
 end
 
 # app/models/device.rb
-class Device < ActiveRecord::Base
+class Device < ApplicationRecord
 end
 
 # Usage
@@ -410,7 +410,7 @@ create_table :devices, id: :uuid, default: 'gen_random_uuid()' do |t|
 end
 
 # app/models/device.rb
-class Device < ActiveRecord::Base
+class Device < ApplicationRecord
 end
 
 # Usage
@@ -434,7 +434,7 @@ end
 execute "CREATE INDEX documents_idx ON documents USING gin(to_tsvector('english', title || ' ' || body));"
 
 # app/models/document.rb
-class Document < ActiveRecord::Base
+class Document < ApplicationRecord
 end
 
 # Usage
@@ -484,7 +484,7 @@ CREATE VIEW articles AS
   SQL
 
 # app/models/article.rb
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   self.primary_key = "id"
   def archive!
     update_attribute :archived, true
diff --git a/guides/source/active_record_querying.md b/guides/source/active_record_querying.md
index ed1c3e7061..4606ac4683 100644
--- a/guides/source/active_record_querying.md
+++ b/guides/source/active_record_querying.md
@@ -25,7 +25,7 @@ Code examples throughout this guide will refer to one or more of the following m
 TIP: All of the following models use `id` as the primary key, unless specified otherwise.
 
 ```ruby
-class Client < ActiveRecord::Base
+class Client < ApplicationRecord
   has_one :address
   has_many :orders
   has_and_belongs_to_many :roles
@@ -33,19 +33,19 @@ end
 ```
 
 ```ruby
-class Address < ActiveRecord::Base
+class Address < ApplicationRecord
   belongs_to :client
 end
 ```
 
 ```ruby
-class Order < ActiveRecord::Base
+class Order < ApplicationRecord
   belongs_to :client, counter_cache: true
 end
 ```
 
 ```ruby
-class Role < ActiveRecord::Base
+class Role < ApplicationRecord
   has_and_belongs_to_many :clients
 end
 ```
@@ -740,7 +740,7 @@ SELECT "articles".* FROM "articles" WHERE (id > 10) ORDER BY id desc LIMIT 20
 The `reorder` method overrides the default scope order. For example:
 
 ```ruby
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   has_many :comments, -> { order('posted_at DESC') }
 end
 
@@ -889,7 +889,7 @@ This behavior can be turned off by setting `ActiveRecord::Base.lock_optimistical
 To override the name of the `lock_version` column, `ActiveRecord::Base` provides a class attribute called `locking_column`:
 
 ```ruby
-class Client < ActiveRecord::Base
+class Client < ApplicationRecord
   self.locking_column = :lock_client_column
 end
 ```
@@ -970,26 +970,26 @@ Active Record lets you use the names of the [associations](association_basics.ht
 For example, consider the following `Category`, `Article`, `Comment`, `Guest` and `Tag` models:
 
 ```ruby
-class Category < ActiveRecord::Base
+class Category < ApplicationRecord
   has_many :articles
 end
 
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   belongs_to :category
   has_many :comments
   has_many :tags
 end
 
-class Comment < ActiveRecord::Base
+class Comment < ApplicationRecord
   belongs_to :article
   has_one :guest
 end
 
-class Guest < ActiveRecord::Base
+class Guest < ApplicationRecord
   belongs_to :comment
 end
 
-class Tag < ActiveRecord::Base
+class Tag < ApplicationRecord
   belongs_to :article
 end
 ```
@@ -1199,7 +1199,7 @@ Scoping allows you to specify commonly-used queries which can be referenced as m
 To define a simple scope, we use the `scope` method inside the class, passing the query that we'd like to run when this scope is called:
 
 ```ruby
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   scope :published, -> { where(published: true) }
 end
 ```
@@ -1207,7 +1207,7 @@ end
 This is exactly the same as defining a class method, and which you use is a matter of personal preference:
 
 ```ruby
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   def self.published
     where(published: true)
   end
@@ -1217,7 +1217,7 @@ end
 Scopes are also chainable within scopes:
 
 ```ruby
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   scope :published,               -> { where(published: true) }
   scope :published_and_commented, -> { published.where("comments_count > 0") }
 end
@@ -1241,7 +1241,7 @@ category.articles.published # => [published articles belonging to this category]
 Your scope can take arguments:
 
 ```ruby
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   scope :created_before, ->(time) { where("created_at < ?", time) }
 end
 ```
@@ -1255,7 +1255,7 @@ Article.created_before(Time.zone.now)
 However, this is just duplicating the functionality that would be provided to you by a class method.
 
 ```ruby
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   def self.created_before(time)
     where("created_at < ?", time)
   end
@@ -1274,7 +1274,7 @@ If we wish for a scope to be applied across all queries to the model we can use
 `default_scope` method within the model itself.
 
 ```ruby
-class Client < ActiveRecord::Base
+class Client < ApplicationRecord
   default_scope { where("removed_at IS NULL") }
 end
 ```
@@ -1290,7 +1290,7 @@ If you need to do more complex things with a default scope, you can alternativel
 define it as a class method:
 
 ```ruby
-class Client < ActiveRecord::Base
+class Client < ApplicationRecord
   def self.default_scope
     # Should return an ActiveRecord::Relation.
   end
@@ -1301,7 +1301,7 @@ NOTE: The `default_scope` is also applied while creating/building a record.
 It is not applied while updating a record. E.g.:
 
 ```ruby
-class Client < ActiveRecord::Base
+class Client < ApplicationRecord
   default_scope { where(active: true) }
 end
 
@@ -1314,7 +1314,7 @@ Client.unscoped.new # => #<Client id: nil, active: nil>
 Just like `where` clauses scopes are merged using `AND` conditions.
 
 ```ruby
-class User < ActiveRecord::Base
+class User < ApplicationRecord
   scope :active, -> { where state: 'active' }
   scope :inactive, -> { where state: 'inactive' }
 end
@@ -1343,7 +1343,7 @@ One important caveat is that `default_scope` will be prepended in
 `scope` and `where` conditions.
 
 ```ruby
-class User < ActiveRecord::Base
+class User < ApplicationRecord
   default_scope { where state: 'pending' }
   scope :active, -> { where state: 'active' }
   scope :inactive, -> { where state: 'inactive' }
@@ -1405,7 +1405,7 @@ Enums
 The `enum` macro maps an integer column to a set of possible values.
 
 ```ruby
-class Book < ActiveRecord::Base
+class Book < ApplicationRecord
   enum availability: [:available, :unavailable]
 end
 ```
@@ -1657,7 +1657,7 @@ a large or often-running query. However, any model method overrides will
 not be available. For example:
 
 ```ruby
-class Client < ActiveRecord::Base
+class Client < ApplicationRecord
   def name
     "I am #{super}"
   end
@@ -1692,7 +1692,7 @@ Person.ids
 ```
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   self.primary_key = "person_id"
 end
 
diff --git a/guides/source/active_record_validations.md b/guides/source/active_record_validations.md
index ec31385077..dd7adf09a2 100644
--- a/guides/source/active_record_validations.md
+++ b/guides/source/active_record_validations.md
@@ -20,7 +20,7 @@ Validations Overview
 Here's an example of a very simple validation:
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, presence: true
 end
 
@@ -80,7 +80,7 @@ method to determine whether an object is already in the database or not.
 Consider the following simple Active Record class:
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
 end
 ```
 
@@ -157,7 +157,7 @@ and returns true if no errors were found in the object, and false otherwise.
 As you saw above:
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, presence: true
 end
 
@@ -175,7 +175,7 @@ even if it's technically invalid, because validations are automatically run
 only when the object is saved, such as with the `create` or `save` methods.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, presence: true
 end
 
@@ -221,7 +221,7 @@ it doesn't verify the validity of the object as a whole. It only checks to see
 whether there are errors found on an individual attribute of the object.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, presence: true
 end
 
@@ -239,7 +239,7 @@ To check which validations failed on an invalid attribute, you can use
 key to get the symbol of the validator:
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, presence: true
 end
 
@@ -285,7 +285,7 @@ the field does exist in your database, the `accept` option must be set to
 `true` or else the validation will not run.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :terms_of_service, acceptance: true
 end
 ```
@@ -297,7 +297,7 @@ It can receive an `:accept` option, which determines the value that will be
 considered acceptance. It defaults to "1" and can be easily changed.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :terms_of_service, acceptance: { accept: 'yes' }
 end
 ```
@@ -309,7 +309,7 @@ and they also need to be validated. When you try to save your object, `valid?`
 will be called upon each one of the associated objects.
 
 ```ruby
-class Library < ActiveRecord::Base
+class Library < ApplicationRecord
   has_many :books
   validates_associated :books
 end
@@ -332,7 +332,7 @@ or a password. This validation creates a virtual attribute whose name is the
 name of the field that has to be confirmed with "_confirmation" appended.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :email, confirmation: true
 end
 ```
@@ -349,7 +349,7 @@ confirmation, make sure to add a presence check for the confirmation attribute
 (we'll take a look at `presence` later on in this guide):
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :email, confirmation: true
   validates :email_confirmation, presence: true
 end
@@ -360,7 +360,7 @@ confirmation constraint will be case sensitive or not. This option defaults to
 true.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :email, confirmation: { case_sensitive: false }
 end
 ```
@@ -373,7 +373,7 @@ This helper validates that the attributes' values are not included in a given
 set. In fact, this set can be any enumerable object.
 
 ```ruby
-class Account < ActiveRecord::Base
+class Account < ApplicationRecord
   validates :subdomain, exclusion: { in: %w(www us ca jp),
     message: "%{value} is reserved." }
 end
@@ -393,7 +393,7 @@ This helper validates the attributes' values by testing whether they match a
 given regular expression, which is specified using the `:with` option.
 
 ```ruby
-class Product < ActiveRecord::Base
+class Product < ApplicationRecord
   validates :legacy_code, format: { with: /\A[a-zA-Z]+\z/,
     message: "only allows letters" }
 end
@@ -409,7 +409,7 @@ This helper validates that the attributes' values are included in a given set.
 In fact, this set can be any enumerable object.
 
 ```ruby
-class Coffee < ActiveRecord::Base
+class Coffee < ApplicationRecord
   validates :size, inclusion: { in: %w(small medium large),
     message: "%{value} is not a valid size" }
 end
@@ -428,7 +428,7 @@ This helper validates the length of the attributes' values. It provides a
 variety of options, so you can specify length constraints in different ways:
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, length: { minimum: 2 }
   validates :bio, length: { maximum: 500 }
   validates :password, length: { in: 6..20 }
@@ -451,7 +451,7 @@ number corresponding to the length constraint being used. You can still use the
 `:message` option to specify an error message.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :bio, length: { maximum: 1000,
     too_long: "%{count} characters is the maximum allowed" }
 end
@@ -483,7 +483,7 @@ WARNING. Note that the regular expression above allows a trailing newline
 character.
 
 ```ruby
-class Player < ActiveRecord::Base
+class Player < ApplicationRecord
   validates :points, numericality: true
   validates :games_played, numericality: { only_integer: true }
 end
@@ -521,7 +521,7 @@ This helper validates that the specified attributes are not empty. It uses the
 is, a string that is either empty or consists of whitespace.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, :login, :email, presence: true
 end
 ```
@@ -531,7 +531,7 @@ whether the associated object itself is present, and not the foreign key used
 to map the association.
 
 ```ruby
-class LineItem < ActiveRecord::Base
+class LineItem < ApplicationRecord
   belongs_to :order
   validates :order, presence: true
 end
@@ -541,7 +541,7 @@ In order to validate associated records whose presence is required, you must
 specify the `:inverse_of` option for the association:
 
 ```ruby
-class Order < ActiveRecord::Base
+class Order < ApplicationRecord
   has_many :line_items, inverse_of: :order
 end
 ```
@@ -568,7 +568,7 @@ This helper validates that the specified attributes are absent. It uses the
 is, a string that is either empty or consists of whitespace.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, :login, :email, absence: true
 end
 ```
@@ -578,7 +578,7 @@ whether the associated object itself is absent, and not the foreign key used
 to map the association.
 
 ```ruby
-class LineItem < ActiveRecord::Base
+class LineItem < ApplicationRecord
   belongs_to :order
   validates :order, absence: true
 end
@@ -588,7 +588,7 @@ In order to validate associated records whose absence is required, you must
 specify the `:inverse_of` option for the association:
 
 ```ruby
-class Order < ActiveRecord::Base
+class Order < ApplicationRecord
   has_many :line_items, inverse_of: :order
 end
 ```
@@ -611,7 +611,7 @@ with the same value for a column that you intend to be unique. To avoid that,
 you must create a unique index on that column in your database.
 
 ```ruby
-class Account < ActiveRecord::Base
+class Account < ApplicationRecord
   validates :email, uniqueness: true
 end
 ```
@@ -623,7 +623,7 @@ There is a `:scope` option that you can use to specify one or more attributes th
 are used to limit the uniqueness check:
 
 ```ruby
-class Holiday < ActiveRecord::Base
+class Holiday < ApplicationRecord
   validates :name, uniqueness: { scope: :year,
     message: "should happen once per year" }
 end
@@ -635,7 +635,7 @@ uniqueness constraint will be case sensitive or not. This option defaults to
 true.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, uniqueness: { case_sensitive: false }
 end
 ```
@@ -658,7 +658,7 @@ class GoodnessValidator < ActiveModel::Validator
   end
 end
 
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates_with GoodnessValidator
 end
 ```
@@ -686,7 +686,7 @@ class GoodnessValidator < ActiveModel::Validator
   end
 end
 
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates_with GoodnessValidator, fields: [:first_name, :last_name]
 end
 ```
@@ -699,7 +699,7 @@ If your validator is complex enough that you want instance variables, you can
 easily use a plain old Ruby object instead:
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validate do |person|
     GoodnessValidator.new(person).validate
   end
@@ -728,7 +728,7 @@ passed to `validates_each` will be tested against it. In the following example,
 we don't want names and surnames to begin with lower case.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates_each :name, :surname do |record, attr, value|
     record.errors.add(attr, 'must start with upper case') if value =~ /\A[[:lower:]]/
   end
@@ -751,7 +751,7 @@ The `:allow_nil` option skips the validation when the value being validated is
 `nil`.
 
 ```ruby
-class Coffee < ActiveRecord::Base
+class Coffee < ApplicationRecord
   validates :size, inclusion: { in: %w(small medium large),
     message: "%{value} is not a valid size" }, allow_nil: true
 end
@@ -764,7 +764,7 @@ will let validation pass if the attribute's value is `blank?`, like `nil` or an
 empty string for example.
 
 ```ruby
-class Topic < ActiveRecord::Base
+class Topic < ApplicationRecord
   validates :title, length: { is: 5 }, allow_blank: true
 end
 
@@ -787,7 +787,7 @@ A `Proc` `:message` value is given two arguments: a message key for i18n, and
 a hash with `:model`, `:attribute`, and `:value` key-value pairs.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   # Hard-coded message
   validates :name, presence: { message: "must be given please" }
 
@@ -818,7 +818,7 @@ new record is created or `on: :update` to run the validation only when a record
 is updated.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   # it will be possible to update email with a duplicated value
   validates :email, uniqueness: true, on: :create
 
@@ -837,7 +837,7 @@ You can also specify validations to be strict and raise
 `ActiveModel::StrictValidationFailed` when the object is invalid.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, presence: { strict: true }
 end
 
@@ -847,7 +847,7 @@ Person.new.valid?  # => ActiveModel::StrictValidationFailed: Name can't be blank
 There is also the ability to pass a custom exception to the `:strict` option.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :token, presence: true, uniqueness: true, strict: TokenGenerationException
 end
 
@@ -871,7 +871,7 @@ to the name of a method that will get called right before validation happens.
 This is the most commonly used option.
 
 ```ruby
-class Order < ActiveRecord::Base
+class Order < ApplicationRecord
   validates :card_number, presence: true, if: :paid_with_card?
 
   def paid_with_card?
@@ -887,7 +887,7 @@ contain valid Ruby code. You should use this option only when the string
 represents a really short condition.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :surname, presence: true, if: "name.nil?"
 end
 ```
@@ -900,7 +900,7 @@ inline condition instead of a separate method. This option is best suited for
 one-liners.
 
 ```ruby
-class Account < ActiveRecord::Base
+class Account < ApplicationRecord
   validates :password, confirmation: true,
     unless: Proc.new { |a| a.password.blank? }
 end
@@ -912,7 +912,7 @@ Sometimes it is useful to have multiple validations use one condition. It can
 be easily achieved using `with_options`.
 
 ```ruby
-class User < ActiveRecord::Base
+class User < ApplicationRecord
   with_options if: :is_admin? do |admin|
     admin.validates :password, length: { minimum: 10 }
     admin.validates :email, presence: true
@@ -930,7 +930,7 @@ should happen, an `Array` can be used. Moreover, you can apply both `:if` and
 `:unless` to the same validation.
 
 ```ruby
-class Computer < ActiveRecord::Base
+class Computer < ApplicationRecord
   validates :mouse, presence: true,
                     if: ["market.retail?", :desktop?],
                     unless: Proc.new { |c| c.trackpad.present? }
@@ -984,7 +984,7 @@ class EmailValidator < ActiveModel::EachValidator
   end
 end
 
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :email, presence: true, email: true
 end
 ```
@@ -1008,7 +1008,7 @@ so your custom validation methods should add errors to it when you
 wish validation to fail:
 
 ```ruby
-class Invoice < ActiveRecord::Base
+class Invoice < ApplicationRecord
   validate :expiration_date_cannot_be_in_the_past,
     :discount_cannot_be_greater_than_total_value
 
@@ -1032,7 +1032,7 @@ custom validations by giving an `:on` option to the `validate` method,
 with either: `:create` or `:update`.
 
 ```ruby
-class Invoice < ActiveRecord::Base
+class Invoice < ApplicationRecord
   validate :active_customer, on: :create
 
   def active_customer
@@ -1053,7 +1053,7 @@ The following is a list of the most commonly used methods. Please refer to the `
 Returns an instance of the class `ActiveModel::Errors` containing all errors. Each key is the attribute name and the value is an array of strings with all errors.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, presence: true, length: { minimum: 3 }
 end
 
@@ -1072,7 +1072,7 @@ person.errors.messages # => {}
 `errors[]` is used when you want to check the error messages for a specific attribute. It returns an array of strings with all error messages for the given attribute, each string with one error message. If there are no errors related to the attribute, it returns an empty array.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, presence: true, length: { minimum: 3 }
 end
 
@@ -1097,7 +1097,7 @@ The `add` method lets you add an error message related to a particular attribute
 The `errors.full_messages` method (or its equivalent, `errors.to_a`) returns the error messages in a user-friendly format, with the capitalized attribute name prepended to each message, as shown in the examples below.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   def a_method_used_for_validation_purposes
     errors.add(:name, "cannot contain the characters !@#%*()_-+=")
   end
@@ -1115,7 +1115,7 @@ person.errors.full_messages
 An equivalent to `errors#add` is to use `<<` to append a message to the `errors.messages` array for an attribute:
 
 ```ruby
-  class Person < ActiveRecord::Base
+  class Person < ApplicationRecord
     def a_method_used_for_validation_purposes
       errors.messages[:name] << "cannot contain the characters !@#%*()_-+="
     end
@@ -1136,7 +1136,7 @@ You can specify a validator type to the returned error details hash using the
 `errors.add` method.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   def a_method_used_for_validation_purposes
     errors.add(:name, :invalid_characters)
   end
@@ -1152,7 +1152,7 @@ To improve the error details to contain the unallowed characters set for instanc
 you can pass additional keys to `errors.add`.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   def a_method_used_for_validation_purposes
     errors.add(:name, :invalid_characters, not_allowed: "!@#%*()_-+=")
   end
@@ -1172,7 +1172,7 @@ validator type.
 You can add error messages that are related to the object's state as a whole, instead of being related to a specific attribute. You can use this method when you want to say that the object is invalid, no matter the values of its attributes. Since `errors[:base]` is an array, you can simply add a string to it and it will be used as an error message.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   def a_method_used_for_validation_purposes
     errors[:base] << "This person is invalid because ..."
   end
@@ -1184,7 +1184,7 @@ end
 The `clear` method is used when you intentionally want to clear all the messages in the `errors` collection. Of course, calling `errors.clear` upon an invalid object won't actually make it valid: the `errors` collection will now be empty, but the next time you call `valid?` or any method that tries to save this object to the database, the validations will run again. If any of the validations fail, the `errors` collection will be filled again.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, presence: true, length: { minimum: 3 }
 end
 
@@ -1207,7 +1207,7 @@ p.errors[:name]
 The `size` method returns the total number of error messages for the object.
 
 ```ruby
-class Person < ActiveRecord::Base
+class Person < ApplicationRecord
   validates :name, presence: true, length: { minimum: 3 }
 end
 
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
 ```
diff --git a/guides/source/plugins.md b/guides/source/plugins.md
index 922bbb4f73..ae8c30515a 100644
--- a/guides/source/plugins.md
+++ b/guides/source/plugins.md
@@ -17,7 +17,7 @@ After reading this guide, you will know:
 This guide describes how to build a test-driven plugin that will:
 
 * Extend core Ruby classes like Hash and String.
-* Add methods to `ActiveRecord::Base` in the tradition of the `acts_as` plugins.
+* Add methods to `ApplicationRecord` in the tradition of the `acts_as` plugins.
 * Give you information about where to put generators in your plugin.
 
 For the purpose of this guide pretend for a moment that you are an avid bird watcher.
@@ -182,7 +182,6 @@ To start out, write a failing test that shows the behavior you'd like:
 require 'test_helper'
 
 class ActsAsYaffleTest < ActiveSupport::TestCase
-
   def test_a_hickwalls_yaffle_text_field_should_be_last_squawk
     assert_equal "last_squawk", Hickwall.yaffle_text_field
   end
@@ -190,7 +189,6 @@ class ActsAsYaffleTest < ActiveSupport::TestCase
   def test_a_wickwalls_yaffle_text_field_should_be_last_tweet
     assert_equal "last_tweet", Wickwall.yaffle_text_field
   end
-
 end
 ```
 
@@ -234,22 +232,22 @@ like yaffles.
 ```ruby
 # test/dummy/app/models/hickwall.rb
 
-class Hickwall < ActiveRecord::Base
+class Hickwall < ApplicationRecord
   acts_as_yaffle
 end
 
 # test/dummy/app/models/wickwall.rb
 
-class Wickwall < ActiveRecord::Base
+class Wickwall < ApplicationRecord
   acts_as_yaffle yaffle_text_field: :last_tweet
 end
-
 ```
 
 We will also add code to define the `acts_as_yaffle` method.
 
 ```ruby
 # yaffle/lib/yaffle/acts_as_yaffle.rb
+
 module Yaffle
   module ActsAsYaffle
     extend ActiveSupport::Concern
@@ -265,7 +263,13 @@ module Yaffle
   end
 end
 
-ActiveRecord::Base.include(Yaffle::ActsAsYaffle)
+# test/dummy/app/models/application_record.rb
+
+class ApplicationRecord < ActiveRecord::Base
+  include Yaffle::ActsAsYaffle
+
+  self.abstract_class = true
+end
 ```
 
 You can then return to the root directory (`cd ../..`) of your plugin and rerun the tests using `rake`.
@@ -308,7 +312,13 @@ module Yaffle
   end
 end
 
-ActiveRecord::Base.include(Yaffle::ActsAsYaffle)
+# test/dummy/app/models/application_record.rb
+
+class ApplicationRecord < ActiveRecord::Base
+  include Yaffle::ActsAsYaffle
+
+  self.abstract_class = true
+end
 ```
 
 When you run `rake`, you should see the tests all pass:
@@ -329,7 +339,6 @@ To start out, write a failing test that shows the behavior you'd like:
 require 'test_helper'
 
 class ActsAsYaffleTest < ActiveSupport::TestCase
-
   def test_a_hickwalls_yaffle_text_field_should_be_last_squawk
     assert_equal "last_squawk", Hickwall.yaffle_text_field
   end
@@ -382,7 +391,13 @@ module Yaffle
   end
 end
 
-ActiveRecord::Base.include(Yaffle::ActsAsYaffle)
+# test/dummy/app/models/application_record.rb
+
+class ApplicationRecord < ActiveRecord::Base
+  include Yaffle::ActsAsYaffle
+
+  self.abstract_class = true
+end
 ```
 
 Run `rake` one final time and you should see: