An Introduction to ActiveRecord

OmbuLabs, August 2015

Active Record is a design pattern

“An object that wraps a row in a database table, encapsulates the database access,

and adds domain logic on that data.”

http://www.martinfowler.com/eaaCatalog/activeRecord.html

One class per table (usually)

Rails Modelclass User < ActiveRecord::Baseend

DatabaseCREATE TABLE `users` ( `id` int(11) NOT NULL auto_increment, `login` varchar(255), `email` varchar(255), `updated_at` datetime default NULL, `created_at` datetime default NULL, PRIMARY KEY (`id`))

One object per table row

One attribute per table column

The ActiveRecord we know

app/models/user.rbclass User < ActiveRecord::Baseend

Creating a user> user = User.new(email: “example@example.com”)# the user doesn’t exist in our database yet. ALL Ruby classes have the `new` method. > user.save! SQL (14.1ms) INSERT INTO `users` (`email`) VALUES (‘example@example.com’)# user is now saved to the database.

> User.create(email: “example@example.com”)SQL (14.1ms) INSERT INTO `users` (`email`) VALUES (‘example@example.com’)

The ActiveRecord we know

app/models/user.rbclass User < ActiveRecord::Baseend

Loading a user> user = User.find(1)User load (0.00514) SELECT * FROM users WHERE (users.id = 1) LIMIT 1=> #<User id: 1, email: “example@example.com”>

Associations1 to 1 has_one & belongs_to

1 to n has_many & belongs_to

n to n has_and_belongs_to_many has_many :through

What can AR return?

ActiveRecord can return a single object, an array of objects, or an Active Relation, among others.

User.find(1) => # Single objectUser.first => # Single objectUser.last => # Single objectUser.all => # Array of objectsUser.count => # IntegerUser.where(id: 1) => # Active Relation

Active Relations & Scopes

You can chain conditions before hitting the database by using `joins`, `where`, `order` and scopes. class User < ActiveRecord::Base scope :not_real, -> { where(“email like ?”, “%example.com%”) }end

> User.not_real=> #<User id: 1, email: “example@example.com”>

> User.not_real.where(id: 2)=> []

THANK YOU!questions?