Single table inheritance (单表继承)
Active Record allows inheritance by storing the name of the class in a column that by default is named “type” (can be changed by overwriting
Base.inheritance_column). This means that an inheritance looking like this:
class Company < ActiveRecord::Base; end class Firm < Company; end class Client < Company; end class PriorityClient < Client; end
When you do
Firm.create(:name => “37signals”), this record will be saved in the companies table with type = “Firm”. You can then fetch this row again using
Company.where(:name => ‘37signals’).first and it will return a Firm object.
If you don’t have a type column defined in your table, single-table inheritance won’t be triggered. In that case, it’ll work just like normal subclasses with no special magic for differentiating between them or reloading the right type with find.
Note, all the attributes for all the cases are kept in the same table. Read more: www.martinfowler.com/eaaCatalog/singleTableInheritance.html