class CreateTests < ActiveRecord::Migration
  def self.up
    create_table :my_tests do |t|
      t.string :type
      t.timestamps
    end
  end

  def self.down
    drop_table :my_tests
  end
end

#MyTest.rb
class MyTest < ActiveRecord::Base
end

#SubTest.rb
class SubTest < MyTest
end

#SubSubTest.rb
class SubSubTest < SubTest
end

#in a console
MyTest.create
SubTest.create
SubSubTest.create

SubTest.find(:all)
 # => SELECT * FROM `my_tests` WHERE ( (`my_tests`.`type` = 'SubTest' OR `my_tests`.`type` = 'SubSubTest' ) ) 

SubTest.find(:all)
 # Same thing:
 # => SELECT * FROM `my_tests` WHERE ( (`my_tests`.`type` = 'SubTest' OR `my_tests`.`type` = 'SubSubTest' ) ) 

ActiveRecord::Base.reset_subclasses
SubTest.find(:all)
 # => SELECT * FROM `my_tests` WHERE ( (`my_tests`.`type` = 'SubTest' ) ) 

CreateTests.down