How should I model these relationships? - ruby-on-rails

Question

I have a post model. Each post has a title and many snippets.
class Post < ActiveRecord::Base
has_many :snippets
end
class Snippet < ActiveRecord::Base
belongs_to :post
attr_accessible :position, :post_id
end
I want to have 4 different types of snippet i.e.:
body (text)
image (string)
code (text)
video (string)
Q1
Should I create four new models (called text, code, video and image) which extend the snippet model like so?:
class Text < Snipppet
attr_accessible :body
end
class Image < Snippet
attr_accessible :image
end
class Video < Snippet
attr_accessible :title
end
class Code < Snippet
attr_accessible code
end
Q2
How can I refer to the content of each snippet in my view when each snippet can be one of 4 different things?
In my view I'd like to put something like this:
- for snippet in #post.snippets
= place the content of the snippet here
Q3
I don't think it sounds like a good idea to have a "type" field on the snippet model as this would possibly lead to strong coupling of the database and the code. Is there some kind of rails magic that will help me out in this situation?

Answer 1

I kinda like the type field, actually. You could use the magic of single table inheritance, but it's well-known to be a fragile system that, surprise, includes a type field, anyway.
And then you could use the polymorphism solution, but seems a bit silly to have four different tables to represent almost exactly the same thing.
Honestly, using a simple type field and changing the interpretation based on that will probably give you the best results and fewest headaches.
As far as what you'd do in your template goes, that you can probably play by ear. One clean solution might be a simple helper to call like snippet_content which, based on the type field, will call the helper snippet_content_text or snippet_content_image or snippet_content_code or snippet_content_video. Or you can just do the if-then branching in the template, or refer to any number of partial templates (though be careful with that, since those can get slow when used unnecessarily).

Answer 2

I like this approach, but I always run into some subtle complications later on.
Create the appropriate views (texts/_text.html.haml, images/_image.html.haml, etc), so you can let Rails handle it for you:
= render #post.snippets
Like it or not: the "type"-field is the way to have Rails magic help you create the proper instances.
Update: polymorphic relations work too, let every snippet have their own tables. Is it a is a relation or a behaves like type of relation? You could say that Images and Texts both behave like snippets. In that case, go for a module named Snippet and mix it in.
class Post < ActiveRecord::Base
has_many :snippets, :polymorphic => true
end
class Text < ActiveRecord::Base
include Snippet
belongs_to :post, :as => :snippet
end
module Snippet
# shared behavior here
end

Answer 3

Many thanks to everyone who answered, but I think I've managed to fix this by using jystewart's rails 3 version of the acts_as_polymorphs gem
Here's what I did:
So let's recall, we have Posts and we have four different types of snippet. So that's 6 different models (post.rb, snippet.rb, code.rb. text.rb, image.rb and video.rb)
Posts have many snippets.
Snippets belong to posts.
Code, text, video and image are all types of snippet
And the big problem is that we don't know what type of object a snippet is, because it can be one of 4 different things.
At first, I tried to do this with Single Table Inheritance, but the objects (code, text, video and image) are, in my opinion, too different from one another for this to work well and I didn't like the fact that it would likely result in lots of empty database cells so I went along the Polymorphic route.
I couldn't get standard polymorphic associations to work here because this is not your usual polymorphic situation. Normally, when dealing with polymorphism, we're talking about something like a comments model, which can attach to multiple other models. The polymorphic entity is always the same thing. But in this case, we're talking about snippets model which can be one of 4 different things. This is not a simple belongs_to situation. Polymorphism was not happening.
Then I stumbled upon this article by m.onkey.org - which is a few years old but he basically explained that this sort of thing needs the acts_as_polymorphs gem.
So the solution was to do the following:
I create 6 models, all extending ActiveRecord::Base
add has_many_polymorphs to the post model
create a polymorphic association called "snippetable" in the snippet model
add some new fields to the snippets table through my migration file
Here's my code:
class Post < ActiveRecord::Base
has_many_polymorphs :snippets, :from => [:codes, :texts, :videos, :images], :through => :snippets
end
class Snippet < ActiveRecord::Base
belongs_to :snippetable, :polymorphic => true
end
class Code < ActiveRecord::Base
# don't have to put anything in here
end
class Text < ActiveRecord::Base
# don't have to put anything in here
end
class Video < ActiveRecord::Base
# don't have to put anything in here
end
class Image < ActiveRecord::Base
# don't have to put anything in here
end
The only other thing we need, is to stick a few new fields in the CreateSnippets migration:
class CreateSnippets < ActiveRecord::Migration
def self.up
create_table :snippets do |t|
t.references :post
t.column :snippetable_id, :integer, :null => false
t.column :snippetable_type, :string, :null => false
t.timestamps
end
end
end
And that's it! Unbelievably I can now go to rails console and do the following:
p = Post.first
p.codes << Code.create(:code=>"This is a code snippet")
p.images << Image.create(:image=>"xyz.png")
p.images << Image.create(:image=>"123.png")
p.codes.count # returns 1
p.images.count # returns 2
p.snippets.count # returns 3 !!!
Yaldi!
Anyway, it's taken me 11 days to fix this, and it REALLY depressed me that I couldn't do it. I hope this helps someone.
Here are some decent reading materials for acts_as_polymorph:
jstewart's rails 3 gem
Pratik Nait (of 37 signals) Blog post on it
What a carry on