Updates person.md

Auto commit by GitBook Editor

Author: Unayung

person.md

@@ -0,0 +1,63 @@
+# 如何讓 Person model 可以指定另一個 person 為 parent
+
+```ruby
+irb(main):001:0> john = Person.create(name: "John")
+irb(main):002:0> jim = Person.create(name: "Jim", parent: john)
+irb(main):003:0> bob = Person.create(name: "Bob", parent: john)
+irb(main):004:0> john.children.map(&:name)
+=> ["Jim", "Bob"]
+```
+如何達成這樣的關連呢？首先我們要知道 association 的 target class 是可以另外指定的，而外鍵的欄位也是可以自己定義
+
+```ruby
+rails g migration add_parent_id_to_person
+# 先增加一個整數欄位 parent_id 到 person table
+
+class Person < ActiveRecord::Base
+  belongs_to :parent, class: Person
+  has_many :children, class: Person, foreign_key: :parent_id
+end
+```
+
+這個問題還可以延伸下去，如果我想達成這樣的效果，需要怎麼實作呢？
+```ruby
+irb(main):001:0> sally = Person.create(name: "Sally")
+irb(main):002:0> sue = Person.create(name: "Sue", parent: sally)
+irb(main):003:0> kate = Person.create(name: "Kate", parent: sally)
+irb(main):004:0> lisa = Person.create(name: "Lisa", parent: sue)
+irb(main):005:0> robin = Person.create(name: "Robin", parent: kate)
+irb(main):006:0> donna = Person.create(name: "Donna", parent: kate)
+irb(main):007:0> sally.grandchildren.map(&:name)
+=> ["Lisa", "Robin", "Donna"]
+```
+
+這邊考的是 has_many ... :through => 這個概念
+
+```ruby
+class Person < ActiveRecord::Base
+  belongs_to :parent, class: Person
+  has_many :children, class: Person, foreign_key: :parent_id
+  has_many :grandchildren, class: Person, through: :children, source: :children
+end
+```
+
+這個範例解釋得很好
+```ruby
+Say you have these models:
+
+Car
+Engine
+Piston
+
+A car has_one :engine
+An engine belongs_to :car
+An engine has_many :pistons
+Piston belongs_to :engine
+
+A car has_many :pistons, through: :engine
+Piston has_one :car, through: :engine
+
+Essentially you are delegating a model relationship to another
+model, so instead of having to call car.engine.pistons, you can
+just do car.pistons
+```