Polymorphic User Table Design Pattern Active Record

Question

I a modeling Users in my rails app

Student < User
Parent < User
Staff < User
User

Every entity has slightly different attributes. I was going to use STI but thinking there might be a better way to model this e.g. originally i had

User
--------
id #primary key
name #common
email #common
type [Student|Parent|Staff]
id      # Student only
school  # Student only
year    # Student only
title   # Parent only
work    # Parent only

i.e Instead I wanted to split this into multiple tables, A User table with all users, then joining via natural keys into user specific tables. Was planning to use NATURAL JOINS e.g. DB would look like

User
--------
id
name
email
type [Student|Parent|Staff]

Student
--------
id
school
year

Parent
-------
title
work

So how do I get active record to do a Natural Join of Student and User table

i.e. I want to be able to do something like

class Student < ActiveRecord::Base  
end           
# would create an entry in Student and User tables.
Student.create(name:'john',school:'green high') 
# get via field in User table and puts out field in Student table
student = Student.where(email:'a@a.com') 
puts student.school

Kinda look for something like http://viralpatel.net/blogs/hibernate-inheritance-table-per-concrete-class-annotation-xml-mapping/

P.S I am about to upgrade to Rails 4 if that makes a difference in syntax would like to know if there is any changes in rails 4 vs rails 3 for this kind of thing.

think about composition over inheritance (http://en.wikipedia.org/wiki/Composition_over_inheritance) — Marian Theisen, Aug 15 '14 at 07:54

score 0 · Answer 1 · edited May 23 '17 at 12:11

This is speculation, but as you've not received much feedback, let me detail how I'd do things:

--

STI

I would keep all entries in a single table and then extend through the classes (how STI's generally work in my experience with Rails)

I would therefore keep your original setup (keep it DRY), and have the following:

#app/models/user.rb
Class User < ActiveRecord::Base
   # fields:
   # id #primary key
   # name #common
   # email #common
   # type [Student|Parent|Staff]
   # school  # Student only
   # year    # Student only
   # title   # Parent only
   # work    # Parent only
end

#app/models/student.rb
Class Student < User
    remove_method "title"
    remove_method "title="

    remove_method "work"
    remove_method "work="
end

#app/models/parent.rb
Class Parent < User
    remove_method "school"
    remove_method "school="

    remove_method "year"
    remove_method "year="
end

#app/models/staff.rb
Class Staff < User
end

This will give you the ability to perform the following:

@student = Student.create(name: "x", school: "y")

Of course, a variety of attributes won't be populated, which lead me to look up this question: Hiding an attribute in an ActiveRecord model

Apparently, you can use the remove_method method to "hide" various attributes in different models. This will give you the ability, albeit as a little hack, to only populate the models with the right data

why remove when you can override and raise an error that is informative about the inheritance structure? This will be clearer a couple of years down the road, when a simple no method error will not be very useful. — pixelearth, Aug 15 '14 at 08:56
Sure - I don't know to be honest. I've not done what I outlined above - just wanted to give a suggestion. You are right, perhaps you could further the answer with an explanation about raising exceptions for `nomethod` errors etc? — Richard Peck, Aug 15 '14 at 08:57

Polymorphic User Table Design Pattern Active Record

1 Answers1