每天一剂Rails良药之Polymorphic Associations - has_many :whatevers

前面的tagging一文中说道acts_as_taggable插件依赖于Rails的多态关联特性，今天我们就来看看它
有时候一个表和多个表关联，比如people和company都有address，我们可以利用Rails的多态关联来实现其功能
(其实我觉得标题应该改为Polymorphic Associations - belongs_to :whatevers才对)

1，Migrations

class AddPeopleCompanyAndAddressTables < ActiveRecord::Migration
  def self.up
    create_table :people do |t|
      t.column :name, :string
    end
    create_table :companies do |t|
      t.column :name, :string
    end
    create_table :addresses do |t|
      t.column :street_address1, :string
      t.column :street_address2, :string
      t.column :city, :string
      t.column :state, :string
      t.column :country, :string
      t.column :postal_code, :string
      t.column :addressable_id, :integer
      t.column :addressable_type, :string
    end
  end

  def self.down
    drop_table :people
    drop_table :companies
    drop_table :addresses
  end
end

我们在addresses表加上addressable_id和addressable_type这两个字段而不是加上person_id或company_id来作为外键关联

2，Models

class Person < ActiveRecord::Base
  has_many :addresses, :as => :addressable
end
class Company < ActiveRecord::Base
  has_many :addresses, :as => :addressable
end
class Address < ActiveRecord::Base
  belongs_to :addressable, :polymorphic => true
end

我们通过addressable来代替Person或Company或whatever，并且指明polymorphic为true
我们可以这样来建立关联:

person = Person.create(...)
address = Address.create(...)
address.addressable = person