Question

I have the following classes mapped with STI:

class Employee < ActiveRecord::Base
end

class StudentEmployee < Employee
  # I'd like to keep university only to StudentEmployee...
end
#Just to make this example easier to understand, not using migrations
ActiveRecord::Schema.define do
    create_table :employees do |table|
        table.column :name, :string
        table.column :salary, :integer
        table.column :university, :string # Only Students

    end
end

emp = Employee.create(:name=>"Joe",:salary=>20000,:university=>"UCLA")

I’d like to prevent the setting of the university field for Employees, but allow it for StudentEmployees. I tried to use attr_protected, but it will only prevent mass setting:

class Employee < ActiveRecord::Base
  attr_protected :university
end

class StudentEmployee < Employee
  attr_accessible :university
end
#This time, UCLA will not be assigned here
emp = Employee.create(:name=>"Joe",:salary=>20000,:university=>"UCLA")
emp.university = "UCLA" # but this will assign university to any student...
emp.save
puts "only Students should have univesities, but this guy has one..."+emp.university.to_s

The problem here is that it will insert in the database a university for simple employees.
Another problem is that I think it would be better to say in the StudentEmployee class that university is an attribute, and not to say in the Employee that university “is not” a a visible attribute… it just goes in the inverse direction of natural abstraction.

Thanks.

Answer 1

I would try something like this:

class Employee < ActiveRecord::Base
  validate :no_university, unless: lambda { |e| e.type === "StudentEmployee" }
  def no_university
    errors.add :university, "must be empty" unless university.nil?
  end
end

It isn’t the prettiest, but it should to work.