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Testing Single Table Inheritance Models in Ruby on Rails

I had a small problem today when trying to do unit testing of single-table inheritance models in Rails, all my tests initially failed and I wasn’t quite sure why. It’s a pretty simple fix but isn’t documented too well anywhere, so here’s the skinny.

My model structure was pretty simple, I had a Content class that acted as a base class for Article, Page, Event, and a bunch of others. So I added a Content class using a model generator, ending up with:

[code]
class Content < ActiveRecord::Base
end
[/code]

Then I added Article and Page classes using the model generator, and changed the default class definitions so that each of those classes inherited from Content. That looks like this:

[code]
class Article < Content
end

class Page < Content
end
[/code]

Then I ran my tests, expecting that the default “test_true” assertions were going to pass. Explosion!

The first problem was that I named my table “content”, not “contents”. Since this isn’t included in the pluralization rules for Rails, my model was trying to find a table called “contents”. This was easily fixed by changing the Content class like this:

[code]
class Content < ActiveRecord::Base
set_table_name "content"
end
[/code]

However, tests were still failing. After quite a bit of googling, it turns out that it was the auto-generated test fixture names that were the problem. First off, the articles.yml and pages.yml file were useless – when testing STI classes, you set up your fixtures in the superclass fixture definition. That is, in my case I only needed content.yml, and if I wanted to set up an Article fixture in there, it would look like this:

[code]
article_one:
id: 1
title: foo
body: this is a body
type: Article
[/code]

Note that I had to set the type property manually to the class name of the subclass object that the fixture is for. Note also the name of the fixtures file – because of my use of a table name for Content that’s not in the default pluralization rules, the fixture file needed to be content.yml rather than the default contents.yml.

One more thing and it all fell into place. All of the generated unit tests had incorrect fixture file names in them – content_test.rb had fixtures :contents, article_test.rb had fixtures :articles, and page_test.rb had fixtures :pages. All of these needed to be changed to fixtures :content before any of the tests would work. Rails apparently looks for a database table name matching the name of the fixtures file, so until I did this my tests were failing when looking for nonexistent tables.

Hopefully that will save someone from the horror of having to trawl the web using the keywords rails sti test.

Posted by Dave Hrycyszyn in All, Head Labs | .