An introduction to ActiveRecord Associations - Training Session

Transcript

1. An introduction to Active Record Associations
2. Take away from this session Active Record Associations Why Associations? The belongs_to Association The has_one Association The has_many Association The has_many :through Association The has_and_belongs_to_many Association Polymorphic Associations
3. Active Record Associations In Rails, an association is a connection between two Active Record models Associations are implemented using macro-style calls Rails supports six types of associations 1. belongs_to 2. has_one 3. has_many 4. has_many :through 5. has_one :through 6. has_and_belongs_to_many
4. Why Associations? Without associations class Customer < ActiveRecord::Base end class Order < ActiveRecord::Base end To add a new order for an existing customer order = Order.create(order_date: Time.now, customer_id: customer.id) To delete a customer orders = Order.where(customer_id: customer.id) orders.each do |order| order.destroy end customer.destroy
5. Why Associations? With associations class Customer < ActiveRecord::Base has_many :orders, dependent: :destroy end class Order < ActiveRecord::Base belongs_to :customer end To add a new order for an existing customer order = customer.orders.create(order_date: Time.now) To delete a customer customer.destroy To fetch all order of a customer customer.orders To fetch customer details of a order order.customer
6. The belongs_to Association A belongs_to association sets up a one-to-one connection with another model, such that each instance of the declaring model "belongs to" one instance of the other model
7. class Order < ActiveRecord::Base belongs_to :customer end The belongs_to Association class CreateOrders < ActiveRecord::Migration def change create_table :orders do |t| t.belongs_to :customer, index: true t.datetime :order_date t.timestamps null: false end end end OR class CreateOrders < ActiveRecord::Migration def change create_table :orders do |t| t.integer :customer_id, index: true t.datetime :order_date t.timestamps null: false end end end Another way to add reference
8. The has_one Association A has_one association also sets up a one-to-one connection with another model, but with somewhat different semantics (and consequences).
9. The has_one Association class Supplier < ActiveRecord::Base has_one :account end class CreateAccounts < ActiveRecord::Migration def change create_table :accounts do |t| t.belongs_to :supplier, index: true t.string :account_number t.timestamps null: false end end end class CreateAccounts < ActiveRecord::Migration def change create_table :accounts do |t| t.integer :supplier_id, index: true t.string :account_number t.timestamps null: false end end end
10. The has_many Association A has_many association indicates a one-to-many connection with another model
11. The has_many Association class Customer < ActiveRecord::Base has_many :orders end class CreateOrders < ActiveRecord::Migration def change create_table :orders do |t| t.belongs_to :customer, index:true t.datetime :order_date t.timestamps null: false end end end
12. 1 The has_many :through Association A has_many :through association indicates that the declaring model can be matched with zero or more instances of another model by proceeding through a third model
13. 1 The has_many :through Association class Physician < ActiveRecord::Base has_many :appointments has_many :patients, through: :appointments end class Appointment < ActiveRecord::Base belongs_to :physician belongs_to :patient end class Patient < ActiveRecord::Base has_many :appointments has_many :physicians, through: :appointments end class CreateAppointments < ActiveRecord::Migration def change create_table :appointments do |t| t.belongs_to :physician, index: true t.belongs_to :patient, index: true t.datetime :appointment_date t.timestamps null: false end end end
14. 1 1 The has_and_belongs_to_many Association A has_and_belongs_to_many association creates a direct many-to-many connection with another model, with no intervening model.
15. 1 The has_and_belongs_to_many Association class CreateAssemblies < ActiveRecord::Migration def change create_table :assemblies do |t| t.string :name t.timestamps null: false end end end class CreateParts < ActiveRecord::Migration def change create_table :parts do |t| t.string :part_number t.timestamps null: false end end end class CreateAssembliesAndParts < ActiveRecord::Migration def change create_table :assemblies_parts, id: false do |t| t.belongs_to :assembly, index: true t.belongs_to :part, index: true end end end
16. 1 1 Polymorphic Associations With polymorphic associations, a model can belong to more than one other model, on a single association
17. 1 Polymorphic Associations class Picture < ActiveRecord::Base belongs_to :imageable, polymorphic: true end class Employee < ActiveRecord::Base has_many :pictures, as: :imageable end class Product < ActiveRecord::Base has_many :pictures, as: :imageable end class CreatePictures < ActiveRecord::Migration def change create_table :pictures do |t| t.string :name t.integer :imageable_id t.string :imageable_type t.timestamps null: false end add_index :pictures, :imageable_id end end class CreatePictures < ActiveRecord::Migration def change create_table :pictures do |t| t.string :name t.references :imageable, polymorphic: true, index: true t.timestamps null: false end end end
18. 1 References http://guides.rubyonrails.org/association_basics.html http://www.slideshare.net/hyoseongchoi/active-record- associations-in-rails http://www.slideshare.net/JamesEdwardGrayII/associations- in-rails