Ruby on rails開發從頭來（五十八）- ActiveRecord基礎（自關聯）

或許存在這樣的情況，在一個表中，一條記錄關聯到表中的另一條記錄，例如，公司中的每個僱員都有上級和下級，而他們同時又是僱員，在Rails中你可以這樣使用Employee類：

class Employee < ActiveRecord::Basebelongs_to :manager,:class_name => "Employee",:foreign_key => "manager_id"belongs_to :mentor,:class_name => "Employee",:foreign_key => "mentor_id"has_many :mentored_employees,:class_name => "Employee",:foreign_key => "mentor_id"has_many :managed_employees,:class_name => "Employee",:foreign_key => "manager_id"end

讓我們使用一些資料，這裡僱員Clem和Dawn都有上級和下級：

Employee.delete_alladam = Employee.create(:id => 1, :name => "Adam")beth = Employee.create(:id => 2, :name => "Beth")clem = Employee.new(:name => "Clem")clem.manager = adamclem.mentor = bethclem.save!dawn = Employee.new(:name => "Dawn")dawn.manager = adamdawn.mentor = clemdawn.save!

現在我們可以通過關聯，來回答“X的下屬是誰？”，“Y的上級是誰？”。

p adam.managed_employees.map {|e| e.name} # => [ "Clem", "Dawn" ]p adam.mentored_employees # => []p dawn.mentor.name # => "Clem"