has_many and single table inheritance

Question

I have a has_many relationship between two entities, Feeds and Posts. I also have specific types of posts, Videos and Photos. This is structured in the database using single table inheritance.

Right now I have my Feed model specifying a has_many relationship between Feeds and Posts (including the subtypes)

class Feed < ActiveRecord::Base
  has_many :posts
  has_many :photos
  has_many :videos

Is there a better, more conventional way to specify this? Or is what I have as simple as it can get?

Permatron

Answer 1

If i understand you correctly you have Posts and posts can be either video or photo. as Jaryl said what you have is probably the easiest to understand/handle however if you wanted to get fancy you could use single table inheritance or polymophic associations.

STI - example (from Agile Web Development with Rails 3rd Edition)

create_table :people, :force => true do |t|
   t.string :type

   #common attributes
   t.string :name
   t.string :email

   #attributes for type=Customer
   t.decimal :balance, :precision => 10, :scale => 2

   #attributes for type=Employee
   t.integer :reports_to
   t.integer :dept

   #attributes for type=Manager
   #none
end

class Person < ActiveRecord::Base
end

class Customer < Person
end

class Employee < Person
   belongs_to :boss, :class_name => "Manager", :foreign_key => :reports_to
end

class Manager < Person
end

So if you create a customer

Customer.create(:name => 'John Doe', :email => '[email protected]', :balance => 78.29)

you can then find it via person

x = Person.find_by_name('John Doe')
x.class #=> Customer
x.email #=> [email protected]
x.balance #=> 78.29
x.some_customer_class_method # will work because the Person.find method returned a Customer object

So you could have

class Post < ActiveRecord::Base
end
class Photo < Post
end
class Video < Post
end

and then you could find them all by Post.all but you would get back Photo and Video objects (and post objects if you have posts that are not photo or video)

don't forget the string :type in your db table

Steiner

#3232389 I'm trying to figure out how to have the STI a child object, so with the given example how would I code "Person belongs_to :company" and "Company has_many :persons" ? – Slim

Answer 2

This is pretty much the simplest you can do.

Well, if photos could be treated the same as videos, then perhaps you could do away with STI and use named scopes to provide accessors to different types of content.

Coronal

Answer 3

I agree that the example in the question is as simple as it gets. It is already using STI and clearly states the associations.

Also, you could rip out the STI later, and split :photos and :videos into their own separate tables without changing the Feed model's code one bit. Score!

Ireful