A DRY way of asserting recurring sets of expectations with RSpec

Often I need to assert a recurring set of expectations in my specs. For instance, I frequently need to test whether an action requires the user to be logged in. To test this I’d need to do a few assertions that together describe the expected behaviour of the system when it asks the user to log in. To DRY this up a little we could put this in a separate method, but that kind of breaks with RSpec’s beautiful expressive DSL. So it would be nice if we can write something like:

@creating_a_group.should require_login

instead of:

describe "creating a group" do
    it "should require login" do
        post :create, :group => { :name => "My group", :description => "My description" }
        response.should redirect_to(login_path)
        flash[:warning].should_not be_nil
        flash[:warning].should include('You need to be logged in')
    end
end

Below is what I came up with after playing around with RSpec. I wrote this before I found out about RSpec’s simple_matcher, with which you can do pretty much the same thing. Still, it might inspire you to see how you can write your own matchers or how to extend RSpec.

Meet Multi Matcher

MultiMatcher (download) is a simple class which can be extended to create your own matchers that assert a set of multiple expectations. Use the expectations class method to set your expectations and optionally override failure_message and negative_failure_message.

To create a matcher that asserts the login requirement behaviour this is all you need:

module MultiMatchers
    class RequireLogin < MultiMatcher
        expectations do
            response.should redirect_to(login_path)
            flash[:warning].should_not be_nil
            flash[:warning].should include('please log in')
        end
    end
    def require_login
        RequireLogin.new(self)
    end
end

Just save this module (e.g. in /lib together with multi_matcher.rb) and include it in your spec environment by adding the following line to spec_helper.rb:

config.include MultiMatchers

With this in place we can use the new matcher anywhere in our specs. Just pass it to the should method on any block of code:

describe "a group" do
    setup do
        @creating_a_group = lambda { post :create, :group => { ... } }
        @destroying_a_group = lambda { delete :destroy, :id => 1 }
    end
    should "require login on create" do
        @creating_a_group.should require_login
    end
    should "require login on destroy" do
        @destroying_a_group.should require_login
    end
end

(Note that saving your actions to instance variables like this isn’t particularly flexible, as it doesn’t allow you to easily perform the same action with different parameters. In my next post I will introduce a trick to overcome this problem.)

Now this makes things a little more DRY in an elegant way. Please leave a comment if you found this a valuable addition to your spec tool set!

Downloads

• multi_matcher.rb