Single Table Inheritance Tricks

Single table inheritance is an extremely powerful and extremely easy thing to do in rails. If you wanted to do this in java you would probably shoot yourself just designing it, but all that pain is relieved inside the wonderful rails framework.

The Scenario
You have the need for users to log into your system with different roles and each of these different roles can have different permissions. But these roles also can have things in common and share permissions. You could add a column into the table called 'role' and then do if(role=="admin") to determine what permissions you should do. But there is an easier solution. Single Table Inheritance.

The Table
First you would create a table called User and put in your common columns eg.

CREATE TABLE `users` ( 
`id` int(11) NOT NULL auto_increment, 
`manager_id` int(11) default NULL, 
`first_name` varchar(255) default NULL, 
`last_name` varchar(255) default NULL, 
`type` varchar(255) default NULL, 
`user_name` varchar(255) default NULL, 
`hash_password` varchar(255) default NULL, 
`email` varchar(255) default NULL, 
`parent_id` int(11) default NULL, 
`created_on` date default NULL, 
PRIMARY KEY (`id`) 
) 

The important column in this Single Table Inheritance is the type column. This column must be a string because what Rails does is it fills this column with the class name.

The Model
The model that goes along with this table is the User model. Now we want to create an Admin model. Admins have the same characteristics of Users, but have different permissions and abilities. To do this all you have to do is extend the User class instead of the ActiveRecord::Base class.
class Admin < User
end
Now you have the power to do calls like Admin.find_all and it will only grab the Admin users. But if you did User.find_all it would include admins in the query. Now you can add methods in the Admin class that are specific for that class only. You can build your permissions into those methods instead of having to dig permissions out of the base User object. You can also take this further. Say you have 3 classes, a User, a Manager, and an Admin. The user has no permissions, the manager has some permissions, and the Admin has both the managers permissions and its own. The way you would do this is to have managers extend user, and Admins extend Manager. All of this is kept in the same table so there is no need to change anything in the database.

Tricks with Extending many classes
there are a few things you can do to help this process out. In the previous scenario if i wanted to use the same 'create' page, I would have to determine what class of user I wanted to create before I saved it. This is a helpful method to put in your User class to help.

def self.factory(type,params) 
type ||= 'User' 
class_name = type 
if defined? class_name.constantize 
return class_name.constantize.new(params)
else 
User.new(params) # or do what you will...
end 
end 

Now when you want to create a user you would do put a role paramter in your page so that the controller can determine what you want:
@user = User.factory(params[:user][:role], params[:user])
@user.save


Another important trick is if you wanted to change that users class or want to know what kind of class he is for display purposes. You would do this by putting these getter and setter methods inside of your User model. def

class_type=(value) 
self[:type] = value 
end 

def class_type 
return self[:type] 
end 

So if you wanted to make a manager into an admin all you would do is

@manager.class_type="Admin" 

and he would magically become an admin.

Hope these tips helped. Leave comments if you have any other suggestions or find a flaw with this tutorial.