How does the Rails' single table inheritance works?

Question

I have a user table, and a teacher that I newly created. The teacher is sub class of user, so, I use scaffold generator to generate the teacher table, than, I modify the model to do teacher is subclass of user. After all that, I did a db:migrate. Then, I go to

http://localhost:3000/teachers/new

It shows an error:

undefined method `teacherSalary' for #<Teacher:0x103331900>

So, my question is what did I do wrong? I want to create a page for doing user register, the user can ONLY be a teacher / student. But I can't add a teacher record ... ... Moreover, I go to

http://localhost:3000/users/new

I want to have a combo box that allow user register their user to be a "teacher" or a "student". But everything seems not work like I expected. What I need to do? Thank you very very much for your help.

Answer 1

Within your database you should have a single table called users. This table should have a string column which by default is called type. If you use another name for this column then you will have to set the inheritance column name manually using self.inheritance_column = "column_name"

Within your application you have three models, User, Student and Teacher. User inherits from ActiveRecord::Base as usual, Student and Teacher both inherit from User.

You should then be able to instantiate new Teacher and Student objects. Internally this works by writing the model name to the type field on the user tables and then when you use Student.find it adds a clause to the SQL to only return rows where the type = 'Student'

You can add shared behaviour to the User class, e.g. validations etc then add additional behaviour to the inherited classes.

A fuller description of how STI works can be found in Martin Fowlers Book(Patterns of Enterprise Application Architecture).

Answer 2

For what you have here , I am not sure if STI is the best way go . STI should generally be used when there is a OO like inheritance and the Models have the same Attribute but different behaviour . In your case Teacher and Student can sure have a few shared attributed , but they are also bound to have different ones as well .
You might want to experiment with a polymorphic association as well .