How to customize Rails 3 STI column name

I have a class named Post:

class Post < ActiveRecord::Base
end

I have a class named Question that inheriting from Post:

class Question < Post
end

and I have a class named Answer that also inheriting from 'Post':

class Answer < Post
end

In Post, I have a column named post_type_id and its' type is Integer.
How do I use STI and specific column name & type to inherit from Post? 0 means Question and 1 means Answer. (0 & 1 is the value of post_type_id in posts table)