An introduction to Ruby on Rails ORM ActiveRecord.

3. Definition © Vita Rara, Inc. Active Record: An object that wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data. -Martin Fowler, Patterns of Enterprise Application Architecture (page 160)

7. ActiveRecord Model Example © Vita Rara, Inc. create_table &quot;persons&quot; do |t| t.string :first_name, last_name t.timestamps end class Person < ActiveRecord::Base end p = Person.new p.first_name = ‘Mark’ p.last_name = ‘Menard’ p.save

10. ActiveRecord::Base.new © Vita Rara, Inc. # Instantiate a new Person, which can be persisted. p = Person. new p.save # Instantiate a new Person, with attributes set based on a # Map, which can be persisted. p = Person. new (:first_name => 'Mark' , :last_name => 'Menard' ) p.save

11. ActiveRecord::Base.create © Vita Rara, Inc. # Immediated create a record in the database. p = Person.create(:first_name => 'Mark' , :last_name => 'Menard' ) # This sets the attributes and calls #save on the instance.

15. Eager Loading: Avoid N+1 Issue © Vita Rara, Inc. <% # Don't put code like this in your view. This is for illustration only! # Find and display order summary of all pending orders for an account. orders = Order.find_pending_by_account(current_account) %> <% orders.each do |order| -%> <%= render :partial => 'order_header' %> <!-- This fires off a query for each order! BAD BAD BAD --> <% order.line_items.each do |line_item| -%> <%= render :partial => 'line_item' %> <% end -%> <% end -%> <% # Better would be orders = Order.find_pending_by_account(current_account, :include => [ :line_items ]) %>

16. Updating Models © Vita Rara, Inc. user = User.find( 1 ) user.first_name = ‘Mark’ user.save # returns true on success user.last_name = ‘Menard’ user.save! # throws an exception if it fails # Immediately update the attributes and call #save # returns true on success user.update_attributes(:first_name => 'John' , :last_name => 'Doe' ) # Immediately update the attributes and call #save! # throws an exception on failure. user.update_attributes!(:password => ‘abccd1234’ )

18. ActiveRecord Associations

21. ActiveRecord Association Examples © Vita Rara, Inc. # Has Many class Order < ActiveRecord::Base has_many :order_line_items end class OrderLineItem < ActiveRecord::Base belongs_to :order end # Has One class Party < ActiveRecord::Base has_one :login_credential end class LoginCredential < ActiveRecord::Base belongs_to :party end

23. Has Many Examples © Vita Rara, Inc. has_many :comments, :order => &quot;posted_on&quot; has_many :comments, :include => :author has_many :people, :class_name => &quot;Person&quot; , :conditions => &quot;deleted = 0&quot; , :order => &quot;name&quot; has_many :tracks, :order => &quot;position&quot; , :dependent => :destroy has_many :comments, :dependent => :nullify has_many :tags, :as => :taggable has_many :subscribers, :through => :subscriptions, :source => :user has_many :subscribers, :class_name => &quot;Person&quot; , :finder_sql => 'SELECT DISTINCT people.* ' + 'FROM people p, post_subscriptions ps ' + 'WHERE ps.post_id = #{id} AND ps.person_id = p.id ' + 'ORDER BY p.first_name'

24. has_many Methods © Vita Rara, Inc. class Firm has_many :clients end firm = Firm.find( 1 ) firm.clients firm.clients << firm.clients.delete firm.clients = firm.client_ids firm.client_ids = firm.clients.clear firm.clients.empty? firm.clients.count firm.clients.find firm.clients.build(:first_name => 'Mark' ) # Like Party.new(:firm_id => firm.id) firm.clients.create(:first_name => 'Mark' ) # Like Party.create(:firm_id => firm.id)

32. Validation Callbacks (cont) © Vita Rara, Inc. class Person < ActiveRecord::Base def validate puts “validate invoked” end def validate_on_create puts “validate_on_create invoked” end def validate_on_update puts “validate_on_update invoked” end end peter = Person.create(:name => “Peter”) # => peter.validate and peter.validate_on_create invoked peter.last_name = “Forsberg” peter.save # => peter.validate_on_update invoked

33. Declarative Validations © Vita Rara, Inc. Rails contains a large number of declarative validations that are applied to classes by convention. Declarative validations free developers from the drudgery of most model validation.

44. Model Life Cycle

49. Cleaning up Attributes Prior to Saving © Vita Rara, Inc. class CreditCard before_validation :cleanup_number private def cleanup_number self.number = number.gsub( /[^0-9]/ , &quot;&quot; ) true # I like to be explicit end end

50. Observers

52. Creating an Audit Trail with an Observer © Vita Rara, Inc. # in config/environment.rb config.active_record_observers = [ :auditor ] # in auditor.rb class Auditor < ActiveRecord::Observer observe User def after_create (model) log_info( &quot;New #{model.class.name} created.&quot; , model) end def after_update (model) log_info( &quot;Update #{model.class.name}&quot; , model) end def after_destroy (model) log_info( &quot;Destroy #{model.class.name}&quot; , model) end private def log_info (model, info) log.info(info) log.info(model.inspect) end end

53. Shameless Self Promotion