Ruby on rails开发从头来（五十八） ActiveRecord基础（自关联）

  或许存在这样的情况，在一个表中，一条记录关联到表中的另一条记录，例如，公司中的每个雇员都有上级和下级，而他们同时又是雇员，在Rails中你可以这样使用Employee类：

 

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

让我们使用一些数据，这里雇员Clem和Dawn都有上级和下级：

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!

现在我们可以通过关联，来回答“X的下属是谁？”，“Y的上级是谁？”。

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

p adam.mentored_employees # => []

p dawn.mentor.name # => "Clem"