ActiveRecord is an object-relational mapping (ORM) pattern that allows developers to interact with a database using objects. It establishes a one-to-one relationship between classes and database tables, with each class representing a database table and objects representing table rows. ActiveRecord provides methods for easily creating, reading, updating, and deleting records in the database. It also handles associations between models and allows chaining conditions to build queries before executing them against the database.

1. An Introduction to
ActiveRecord
OmbuLabs, August 2015

2. 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

3. One class per table (usually)
Rails Model
class User < ActiveRecord::Base
end
Database
CREATE 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`)
)

4. One object per table
row

5. One attribute per table
column

6. The ActiveRecord we know
app/models/user.rb
class User < ActiveRecord::Base
end
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’)

7. The ActiveRecord we know
app/models/user.rb
class User < ActiveRecord::Base
end
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”>

8. Associations
1 to 1
has_one & belongs_to
1 to n
has_many & belongs_to
n to n
has_and_belongs_to_many
has_many :through

9. What can AR return?
ActiveRecord can return a single object, an array
of objects, or an Active Relation, among others.
User.find(1) => # Single object
User.first => # Single object
User.last => # Single object
User.all => # Array of objects
User.count => # Integer
User.where(id: 1) => # Active Relation

10. 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)
=> []

11. THANK YOU!
questions?