Using Inheritance with RoR

Question

I'm working on my first Rails application (my first Ruby app, for that matter) and I have a best practice question about inheritance. I have to be able to upload physical media and I want to store the physical properties separately in the database so that certain attributes can be queried outside of the application as easily as within in. To manage this, I have an images table and a binaries table. The former stores metadata specific to images, of course, while the latter stores physical file metadata that will apply to a range of file types. Once I have my approach solidified, I'll add a swfs table, a videos table and perhaps others.

At the moment, I have a BinaryObserver class captures the before_create callback of an image and uploads the physical file - the binary. This is working fine, but I'm wondering about other approaches. Specifically, I'm wondering if I couldn't (and shouldn't) establish a basic inheritance model where Binary extends ActiveRecord::Base and Image extends Binary.

As it stands right now, Image belongs_to Binary and Binary has_one Image. Since there's an obvious is-a relationship, though, should I use inheritance? Does rails even support such a thing when interacting with a database? This might be the best way to reduce the amount of work required to support a new file type.

Would appreciate any thoughts or suggestions on this. I'm experimenting with the language and the framework, so I'm trying to learn best practices before I get in too deep.

Thanks.

Answer 1

From the DB point of view.. You don't need the binaries table... if you save the binary data in the database, it's just a property- like date.. you wouldn't make dates table :)

To say that Image extends Binary because Image "is-a" Binary is not accurate, Image "is-made-of" binary, like Person made of Flesh (you wouldn't say Person extend Flesh)

Anyway, there is a plugin for attachments: attachment_fu, use it :)

Answer 2

Fortunately, there is a very nice single-table inheritance built into rails.

Unfortunately, what you are looking for is multi-table and support is pretty bleak.

Check out some examples on how people have implemented such:

http://zackham.com/2008/8/21/multiple-table-inheritance

http://github.com/sava/class%5Ftable%5Finheritance/tree/master

http://github.com/rwl4/inherits%5Ffrom/tree/master

Or you could just switch to single-table inheritance (though that defeats your previous comment):

class Binary < ActiveRecord::Base
end

class Image < Binary
end

and add this to your Binary model:

type VARCHAR(20) NOT NULL

Examples:

@binaries = Binary.find(:all)

@images = Image.find(:all)

Answer 3

If you are writing production code and not just testing out stuff, then I highly recommend attachment_fu or similar way to handle this situation.