Rails 中的多表继承
多表继承
什么是多表继承
Multiple Table Inheritance 多表继承就是各个子类有单独的表，但是通过父类来共享通用的方法。
何时使用多表继承
- 子类之间的 Fields/Columns 差异极大，但是需要共享类似的行为
- 多表继承可以一定程度上解决单表过大的问题（对此我个人持反对意见，当 MTI 分表之后造成单表体积巨大，通过分片 sharding 来解决表的体积时，MTI 让你将来的工作更麻烦）
具体实现
class Account < ActiveRecord::Base
def withdraw(amount)
# ...
end
def deposit(amount)
# ...
end
def close!
update_attribute!(closed_at, Time.current)
end
end
# These three types of accounts inherit from a common base class
# so they can all share a common `#close` method. However, each
# class has its own class-specific methods as well as their own
# tables.
class CorporateAccount < Account
set_table_name "corporate_accounts"
def close!
inform_share_holders!
super
end
end
class SmallBusinessAccount < Account
set_table_name "small_business_accounts"
def close!
inform_mom_and_pop!
super
end
end
class PersonalAccount < Account
set_table_name "personal_accounts"
def close!
inform_dude_or_dudette!
super
end
end
# As such, we need to create three tables. A very un-DRY approach.
class CreateCorporateAccounts < ActiveRecord::Migration
def change
create_table :corporate_accounts do |t|
t.float :balance
t.string :tax_identifier
t.date_time :closed_at
# ... more column fields #
t.timestamps
end
end
end
class CreateSBAccounts < ActiveRecord::Migration
def change
create_table :small_business_accounts do |t|
t.float :balance
t.string :tax_identifier
t.date_time :closed_at
# ... more column fields #
t.timestamps
end
end
end
class CreatePersonalAccounts < ActiveRecord::Migration
def change
create_table :personal_accounts do |t|
t.float :balance
t.string :tax_identifier
t.date_time :closed_at
# ... more column fields #
t.timestamps
end
end
end
抽象类 abstract_class
当你有几个子类要继承同一个父类，但是父类并不存在对应的 table，可以在父类声明 abstract_class，在子类声明各自的 table_name。
class Shape < ActiveRecord::Base
self.abstract_class = true
end
Polygon = Class.new(Shape)
Square = Class.new(Polygon)
Shape.table_name # => nil
Polygon.table_name # => "polygons"
Square.table_name # => "polygons"
Shape.create! # => NotImplementedError: Shape is an abstract class and cannot be instantiated.
Polygon.create! # => #<Polygon id: 1, type: nil>
Square.create! # => #<Square id: 2, type: "Square">
