subclasses_issue.rb

class CreateUsers < ActiveRecord::Migration
  def self.up
    create_table :users do |t|
      t.string :type
      t.string :email

      t.timestamps
    end
  end

  def self.down
    drop_table :users
  end
end

# user.rb
class User < ActiveRecord::Base
end

# standard_user.rb
class StandardUser < User
end

# admin_user.rb
class AdminUser < StandardUser
end

# script/console
>> StandardUser.count
# log/development.log
# SELECT count(*) AS count_all FROM "users" WHERE ( ("users"."type" = 'StandardUser' ) ) 
>> StandardUser.send(:subclasses).map(&:name)
=> []

>> AdminUser # just force to load the class
=> AdminUser(id: integer, type: string, email: string, created_at: datetime, updated_at: datetime)

>> StandardUser.send(:subclasses).map(&:name)
=> ["AdminUser"]
>> StandardUser.count
# log/development.log
# SELECT count(*) AS count_all FROM "users" WHERE ( ("users"."type" = 'StandardUser' OR "users"."type" = 'AdminUser' ) )