Single Table Inheritance in Rails

I couldn’t find too much information on Rails awesome built in Single Table Inheritance so I thought I’d write this up for anyone who is interested.

What is Single Table Inheritance?

In a nutshell, STI allows you to create subclasses of a particular database table. Using a single table, you can cast rows to specific objects that extend the base model.

How to create STI relationships in Rails

Lets say we have a model Computer

class Computer < ActiveRecord:Base
  # in app/models
  # Fields:
  #   String name
  #   String owner
  #   String manafacturer
  #   String color

  def default_browser
    "unknown!"
  end 
end

Now, we want to differentiate between Macs and PCs. It doesn’t really make sense to make a different table for each, since they both have pretty much the same columns. Instead we can create a new column, type, which tells Rails to use STI on Computer. Lets look at what the models might look like.

class Computer < ActiveRecord:Base
  # in app/models
  # Fields:
  #   String name
  #   String owner
  #   String manafacturer
  #   String color
  #   String type

  def default_browser
    "unknown!"
  end 
end

class Mac < Computer
  # in app/models
  # this is for Computers with type="Mac"
  before_save :set_color

  # Lets say all macs are silver, no point setting these ourselves
  def set_color
    self.color = "silver"
    self.manafacturer = "apple"
  end

  # Lets overwrite the default_browser method
  def default_browser
    "safari"
  end
end

class PC < Computer
  # in app/models  

  # Lets overwrite the default_browser method
  def default_browser
    "ie =("
  end
end

Anytime Rails opens up the computer object, it looks for the subclass corresponding to type. For instance,type="CoolComputer" corresponds to model CoolComputer < Computer.

How to use STI Models

To create a new mac, you can do:

m = Mac.new
m.name = "kunal's mac"
m.owner = "kunal"
m.save
m # => #<Mac id: 1, name: "kunal's mac", owner: "kunal", manafacturer: "apple", color: "silver", type: "Mac", ...>

Whats even cooler is ActiveRecord queries. Lets say we want all the computers

Computer.all # => [#<Mac id: 1, name: "kunal's mac", owner: "kunal", manafacturer: "apple", color: "silver", type: "Mac", ...>, #<Mac id: 2, name: "anuj's mac", owner: "anuj", manafacturer: "apple", color: "silver", type: "Mac", ...>, #<PC id: 3, name: "bob's pc", owner: "bob", manafacturer: "toshiba", color: "blue", type: "PC", ...>]

Yup, it automatically gives you the correct objects! You can find out the type of a particular object by calling.type, is_a? or .class

Computer.first.type == Mac # true
Computer.first.is_a? Mac # true
Computer.first.class == Mac # true

If we only want Macs, we can do

Mac.all

Custom Inheritance Column

If you want to use another column instead of type to use for STI, you can simply add this to the top of your model:

set_inheritance_column 'whatever_you want'

Note: If you have a database column named type, you can turn off Single Table Inheritance by changing the inheritance column to something other than type.

Organizing This in Rails

After using STI, I ended up with a bloated models folder because all of the many custom sub models I created were in the models folder. To solve this, I created a folder in models to store all of my computer specific models

* app
*   models
*     computer.rb
*     computers
*       pc.rb
*       mac.rb

Rails doesn’t automatically open subfolders in the models folder, so I added in config/application.rb:

# Load Subfolder Models
config.autoload_paths += Dir[Rails.root.join('app', 'models', '{**}')]

Cheers,

Kunal

http://blog.thirst.co/post/14885390861/rails-single-table-inheritance