How would you test observers with rSpec in a Ruby on Rails application?

Question

Suppose you have an ActiveRecord::Observer in one of your Ruby on Rails applications - how do you test this observer with rSpec?

Answer 1

Disclaimer: I've never actually done this on a production site, but it looks like a reasonable way would be to use mock objects, should_receive and friends, and invoke methods on the observer directly

Given the following model and observer:

class Person < ActiveRecord::Base
 def set_status( new_status )
  # do whatever
 end
end

class PersonObserver < ActiveRecord::Observer
 def after_save(person)
  person.set_status("aha!")
 end
end

I would write a spec like this (I ran it, and it passes)

describe PersonObserver do
 before :each do
  @person = stub_model(Person)
  @observer = PersonObserver.instance
 end

 it "should invoke after_save on the observed object" do
  @person.should_receive(:set_status).with("aha!")
  @observer.after_save(@person)
 end
end

Answer 2

You are on the right track, but I have run into a number of frustrating unexpected message errors when using rSpec, observers, and mock objects. When I am spec testing my model, I don't want to have to handle observer behavior in my message expectations.

In your example, there isn't a really good way to spec "set_status" on the model without knowledge of what the observer is going to do to it.

Therefore, I like to use the "No Peeping Toms" plugin. Given your code above and using the No Peeping Toms plugin, I would spec the model like this:

describe Person do 
  it "should set status correctly" do 
    @p = Person.new(:status => "foo")
    @p.set_status("bar")
    @p.save
    @p.status.should eql("bar")
  end
end

You can spec your model code without having to worry that there is an observer out there that is going to come in and clobber your value. You'd spec that separately in the person_observer_spec like this:

describe PersonObserver do
  it "should clobber the status field" do 
    @p = mock_model(Person, :status => "foo")
    @obs = PersonObserver.instance
    @p.should_receive(:set_status).with("aha!")
    @obs.after_save
  end
end

If you REALLY REALLY want to test the coupled Model and Observer class, you can do it like this:

describe Person do 
  it "should register a status change with the person observer turned on" do
    Person.with_observers(:person_observer) do
      lambda { @p = Person.new; @p.save }.should change(@p, :status).to("aha!)
    end
  end
end

99% of the time, I'd rather spec test with the observers turned off. It's just easier that way.