[rails] Polymorphic Associations

[Fletch]

I'm planning an application that would store different types of content. Some attributes would be used by all content types (title, description etc.), other attributes would be specific to the content type (eg. a photo would have a thumbnail_url).

I'd like to avoid single table inheritance, because additional content types are likely to be added overtime so im uncomfortable adding additional attributes to the content table which wont be used by all entities.

Therefore, I'd like a base table that stores the attributes shared by all content types, and then additional tables for the extended data.

For example the base 'content' table would contain the following rows:

Content Table

title
description
created_on
extended_type

Then the 'article', 'photo' and 'audio' tables would contain the extended data for each appropriate type.

Article Table

full_text

Photo Table

full_url
thumbnail_url

Audio Table

length
artist
album

From what I've read this appears to be possible using Rails' polymorphic associations but I'm a bit confused on how to implement this. Would this be achieved using something familiar to the following?

class link < ActiveRecord::Base
  belongs_to :content
  belongs_to :extendedinfo, :polymorphic => true
end

class Content < ActiveRecord::Base
  has_one :link
  has_one :extendedinfo, :through => :link
end

class Article < ActiveRecord::Base
  has_one :link, :as => :extentioninfo
  has_one :content, :through => :link
end

class Photo < ActiveRecord::Base
  has_one :link, :as => :extentioninfo
  has_one :content, :through => :link
end

class Audio < ActiveRecord::Base
  has_one :link, :as => :extentioninfo
  has_one :content, :through => :link
end

Am I on the right track here, or is there another way of achieving similar? Thanks.

[thebasti]

Hi,

You are nearly there.

First of all, as far as I can see, you don't need link model/table. Each Content entry will be related to only one Article, Photo, or Audio entry. So it's enough to add two columns to Content table:

extendedinfo_id
extendedinfo_type

There's a tutorial doing something similar to what you need on Rails Wiki - HowToUsePolymorphicAssociations.

That being said, I am not sure that you really need separate models for each type of content. If I understand correctly, each content type will basically act the same and the only difference will be what data set they have. Generally speaking you need different models only when those models need to act differently. I know that you don't want to use STI, but there's an article 'STI abuse' that discusses when creating new model is a good thing. In you case you might want to create one model with type column which you can then use to know which attributes model has.

Of course this all depends on what you really want to achieve and what's the "big picture", in some cases it's good to have separate models (e.g. if they will become more different over time).

Hope this helps.

[Fletch]

Thanks for your help.

I read HowToUsePolymorphicAssociations before posting but that caused some additional confusion as it's dealing with one-to-many relationships rather than one-to-one relationships.

So would my content table look like the following:

Content Table

id
title
description
created_on
extended_id
extended_type

with a content base class:

models/content.rb

class Content < ActiveRecord::Base
	has_one :extended, :polymorphic => true
end

Would the Image class then extend ActiveRecord::Base with a belongs_to :content, or would it extend the Content class?

models/image.rb

class Image < ActiveRecord::Base
	belongs_to :content
end

or

models/image.rb

class Image < Content
end

[thebasti]

Fletch,

Sorry, I looked more closely at your code and I realized that you have got it wrong. You turned around what relation needs to be polymorphic.

You need something like:

class Content < ActiveRecord::Base
	belongs_to :extended, :polymorphic => true
end
 
class Image < ActiveRecord::Base
	has_one :content, :as => :extended
end

You should also take a look (if you didn't already) at Rails API explanation of Polymorphic Associations (there's a separate section).

[Fletch]

Excellent! Thank you very much for your help hebasti, I have things up and running now.

I presume you can't eager load the extended information when finding all content items eg...

Can you eager load polymorphic relationships when finding from the base class? For example...

Content.find(:all, :include => :extended)

The above doesn't seem to work (which is understandable) but you can eager load the additional information when finding a certain content type.

Image.find(:all, :include => :content)

Not a huge problem anyway, the pages will be heavily cached.

[thebasti]

As far as I know you can't eager load the polymorphic relationships when finding from the base class. As you said, it's understandable since you don't know upfront what tables you should join it to.