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Unit Testing CouchDB Views with Couch Potato

October 30, 2009 by alex

I just released Couch Potato 0.2.14 and amongst other things it has a new feature i think is pretty neat: you can unit test your (JavaScript) views using RSpec and Ruby.

You can declare a view in Couch Potato like this:

class Comment
  property :post_id
  view :by_post_id, :key => :post_id
end

This will generate a pair of map/reduce function and push them to Couch Potato. The map function looks something like this:

[javascript] function(doc) { if(doc.ruby_class == ‘Comment’) { emit(doc.post_id, 1); } } [/javascript]

And here’s a unit test for that function:

describe Comment, 'by_post_id' do
  it "should map to the post_id" do
    Comment.by_post_id.should map({:ruby_class => 'Comment', :post_id => 3}).to([3, null])
  end
end

As you can see all you have to do is pass a Ruby Hash that looks like your CouchDB document and the expected results. You can also pass in multiple results if you expect your map function to emit multiple key/value pairs.

Testing a reduce function works the same way:

describe Comment, 'by_post_id' do
  it "should reduce to the number of comments" do
    Comment.by_post_id.should reduce([], [1, 1, 1]).to(3)
  end
end

For testing re-reducing you simply call .should rereduce(...).to(...).

How it works

So how come you can test JavaScript functions in pure Ruby? Well, by stealing other people’s tricks. I recently contributed a few patches to mustache.js which is a new templating library ported to JavaScript by @janl. It has a test runner currently implemented in Ruby which generates JavaScript code on the fly, runs it using Spidermonkey and reads back the results. I have added a few steps to this process:

  1. A custom RSpec matcher collects the map function, a Ruby Hash representing the input and the expected output
  2. The input is converted to JavaScript using the JSON gem
  3. JavaScript code is generated that runs the document through the map function and prints the resulting JSON.
  4. The Ruby code runs spidermonkey, collects the output and parses it back to Ruby using again the JSON gem
  5. The results are compared to the expected values.

You can see how it works by looking at the code for the RSpec matchers.

I think this addition lowers the barrier to test your views quite a bit. Although I’m usually not a big fan of “one language to rule them all” and love writing JavaScript, being able to write all the necessary tests in Ruby when working on a Ruby project makes things way easier.