ruby-on-rails - unknown attribute errror when implementing rails multiple table inheritance

Question

I followed the post http://techspry.com/ruby_and_rails/multiple-table-inheritance-in-rails-3/ to implement the multiple table inheritance with Rail 4. I have three models: user, applicant and tutor. Here is my code:

class User < ActiveRecord::Base
  belongs_to :student, :polymorphic => true
end

class Tutor < ActiveRecord::Base
  acts_as_student
end

class Applicant < ActiveRecord::Base
  acts_as_student
end

# in the /lib/student_module.rb
module Student
  def acts_as_student
    include InstanceMethods
    has_one :user, :as => :student, :autosave => true, :dependent => :destroy
    alias_method_chain :user, :build

    user_attributes = User.content_columns.map(&:name) #<-- gives access to all columns of Business
    # define the attribute accessor method
    def student_attr_accessor(*attribute_array)
      attribute_array.each do |att|
        define_method(att) do
          user.send(att)
        end
        define_method("#{att}=") do |val|
          user.send("#{att}=",val)
        end
      end
    end
    student_attr_accessor *user_attributes #<- delegating the attributes
  end

  module InstanceMethods
    def user_with_build
      user_without_build || build_user
    end
  end
end

The User Table has username, email attributes.The Tutor table has first_name,last_name,intro,program,entry_year attributes. In the rails console, I got

tutor = Tutor.new => #<Tutor id: nil, first_name: nil, last_name: nil, intro: nil, created_at: nil, updated_at: nil, entry_year: nil, program: nil> 
tutor.username 
=> ActiveRecord::UnknownAttributeError: unknown attribute: student_id

I found the error was from the student_attr_accessor method. How should I fix it? Thanks!

Answer 1

我发现我忘记在用户模型中声明一个外键列和一个类型列。要解决此问题，只需运行如下迁移：

def change
    add_column :users, :student_id,:integer
    add_column :users, :student_type,:string
  end