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Testing Rails Partials
One important metric, under Test Driven Development, is the distance between a test case and its target code. Test cases use assertions to observe events inside programs. If a test case requires more than a few hops to reach its target event, the intermediate methods can add noise to the test's signal.
Some architectures make decoupling test cases very hard. This post develops a fix for an icky Rails problem - testing one small partial .rhtml file embedded in a huge web page.
Rails View Testing
Rails projects can test web pages by rendering them to HTML, then diverting them into test cases. Rails functional tests can get controller actions, then parse web pages, returned in @response.body, to match important details.
(If your web pages are pure XHTML [a very good idea], you can test them with assert_xpath. If they are not, call assert_tidy before assert_xpath.)
Anything your production code pushes into a web page, with <%= %> eRB tags, a test should pull out, using assert_match, assert_xpath, or assert_select.
However, such tests can be noisy. A test that detects an important number, such as 42 in an input field, should not trip over any irrelevant 42s, such as a nearby <img width='42'>. When tests run closer to their tested code, their signal gets stronger.
Rails can generate HTML by pushing .rhtml files (or .html.erb files) together with layout files and partial files. A partial is Rails's unit of HTML reuse. A Rails View can render a partial and insert it into its hosting HTML like this: <%= render :partial => "photos/show" %>
Test Driven Development works best when each test case targets one aspect of a class's interface. So this post will demonstrate a simple and direct way to test a partial without testing the Views, layouts, and Controller actions surrounding it. On very complex projects, this technique keeps your partials decoupled.
This is the Photo Gallery project from [1]Ajax on Rails, by Scott Raymond. I upgraded it to use Rails 2.1, yet these techniques all work freely with any Ruby on Rails version >1.4. Then I added a simple test to its action that shows a gallery of thumbnails:
require File.dirname(__FILE__) + '/../test_helper' require 'albums_controller' require 'assert_xpath' require 'assert2'
class AlbumsController; def rescue_action(e) raise e end; end
class AlbumsControllerTest < ActionController::TestCase include AssertXPath fixtures :albums, :photos # add a couple real fixtures here first!
def test_show album = albums(:first) get :show, :id => album.id
assert_xpath :div, :photos, 'find <div id="photos">' do assert_xpath :'ul/li', album.photos.first.id, 'finds <ul><li id="999">' do assert{ assert_xpath(:a)[:onclick] =~ /Photo.show/ } end end end
end
The line with get :show, :id => album.id simulates a user hitting the show action with the id of a photo album. The page comes back in the secret variable @response.body with the rendered HTML.
An XPath DSL
The first assert_xpath converts that HTML into an XML document. The notation :div, :photos is one of assert_xpath's Domain Specific Language shortcuts. It expands to the complete XPath '//div[ @id = "photos" ]'. You could write all that too, if you wanted.
When assert_xpath's first argument is a 'string', it evaluates as raw XPath. When it's a :symbol, assert_xpath tacks a // on the beginning (or the equivalent), meaning "seek any such node at or below the current node".
Both forms of assert_xpath return only one node - the first one encountered.
The last argument to assert_xpath is a diagnostic string. When assert_xpath fails, it prints out this string, decorated with the current HTML context.
When you call assert_xpath with a block, it narrows that context to that block. Any assert_xpath calls inside that block can only match HTML elements inside that container.
If the line assert_xpath :'ul/li'... failed, it would spew out only the contents of <div id='photos'>. This is very important in Web development, because a complete HTML page could be several screens long. Most of it would not relate to the tested feature.
In summary, that test case detects this HTML: ...<div id='photos'> <ul> <li id='520095529'> <a onclick='Photo.show...'>...</a> </li> </ul> </div>...
I replaced the elements it did not detect with ... ellipses. Further assertions could easily pin down their contents, if they were important.
Cut to the Chase
The code which generated that HTML looks like this:
<div id="photos"><%= render :partial => "photos/index" %></div>
That looks mostly harmless, but imagine if all the other show.rhtml business around it were heavy and expensive; fraught with side-effects. Imagine if we needed to add an important feature into that partial, requiring many test cases. Each test case would have to call extra code to support those side-effects. Expensive test setup is a design smell - it indicates code that's too coupled. When tests run close to their target code, they help it decouple.
Here's how to test that partial directly:
class ApplicationController def _renderizer; render params[:args]; end end
class ActionController::TestCase # or Test::Unit::TestCase, for Rails <2.0 def render(args); get :_renderizer, :args => args; end end
... def test_photo_index album = albums(:first)
render :partial => 'photos/index', :locals => { :@album => album }
assert_xpath :'ul/li', album.photos.first.id, 'finds <ul><li id="999">' do assert{ assert_xpath(:a)[:onclick] =~ /Photo.show/ } end end
That wasn't too horrifying now was it?
In general, Rails's internal architecture can be labyrinthine. That's the price of incredible flexibility. Writing your own copy of render is hard, because a test using ActionController::TestCase does not yet have an ActiveView context. The test method get must concoct one using the same procedures as a real web hit.
To bypass this problem, we first add a secret action to all controllers - _renderizer. Because Ruby is extensible, the runtime application never sees this method. It only appears under test. We implement render by packing up its arguments, passing them thru get to _renderizer, and letting it call render.
The benefit is our test case requires one less assertion. A more complex application could have avoided much more cruft there. And if our assert_xpath failed, now, its diagnostic would report only the partial's own contents.
And the mock render can generate any other View-level thing, isolating it for test. For example, it could test that our application layout has a link called "Gallery", like this:
def test_layout render :layout => 'application', :nothing => true assert_xpath :'a[ . = "Gallery" ]' end
This lightweight solution to a tricky Rails problem illustrates how Ruby applications in general, and Rails in particular, reward thinking outside the box.
References
1. http://examples.oreilly.com/9780596527440/Ajax_on_Rails_Example_Applications.zip