Rails Partial Tests

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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

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