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，公司中的每個雇員都有上級和下級，而他們同時又是雇員，在Rails中你可以這樣使用Employee類：

class Employee < ActiveRecord::Base
belongs_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

讓我們使用一些數據，這裡雇員Clem和Dawn都有上級和下級：

Employee.delete_all
adam = Employee.create(:id => 1, :name => "Adam")
beth = Employee.create(:id => 2, :name => "Beth")
clem = Employee.new(:name => "Clem")
clem.manager = adam
clem.mentor = beth
clem.save!
dawn = Employee.new(:name => "Dawn")
dawn.manager = adam
dawn.mentor = clem
dawn.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"