Why does this simple RSpec string equality spec fail?

Question

Given this class:

class First
  def to_s ; "Hello World" ; end
end

and this spec:

require 'first'

describe First do
  describe "#to_s" do
    it { should == "Hello World" }
  end
end

I get:

Failures:

  1) First#to_s 
     Failure/Error: it { should == "Hello World" }
       expected: "Hello World"
            got: Hello World (using ==)
       Diff:
       @@ -1,2 +1,2 @@
       -"Hello World"
       +Hello World
     # ./spec/first_spec.rb:5:in `block (3 levels) in <top (required)>'

But I would expect this to pass. My questions are:

How would a passing spec look like?
Why is this spec not passing, specifically?

About the rails tags: I didn't put them there, because it is actually not a Rails-specific topic. But I won't remove them either, now that they are there, because of the people interested in the topic probably 80% are Rails devs anyway. — Felix Rabe
– Felix Rabe, Commented Mar 20, 2012 at 7:15

Gareth · Accepted Answer · 2012-03-19 22:30:10Z

4

Your outer describe block:

describe First do

sets the subject of the inner examples to be an instance of First. That is, all of the its you're describing will be a First object. The text #to_s example doesn't instruct rspec to call to_s on the object.

Since First.new is not equal to "Hello World" the spec fails.

However, since rspec does call to_s on the object to output it in the failure description, "Hello World" does appear there. You could try the following to ensure that the string conversion is being tested:

its(:to_s) { should == "Hello World" }

answered Mar 19, 2012 at 22:30

Gareth

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1 Comment

Felix Rabe Over a year ago

Thank you @Gareth and @apneadiving! Two very good answers, though I prefer this one. In summary: Q1, "How...?" -- The passing spec that I intended uses its (see answer's example) and I can get rid of the inner describe level. Q2, "Why...?" -- I thought using describe "#blah" would set the subject, but that is not the case. Instead, the subject is still First.new.

apneadiving · Accepted Answer · 2012-03-19 22:30:09Z

2

I've never used #method_name to get the result evaluated automatically. I prefer verbosity to be sure of what I'm doing.

Generally, I pick one from these:

describe First do
  specify { First.new.to_s.should == "Hello World" }
end

Or:

describe First do
  describe "#to_s" do
    subject { First.new.to_s }
    it { should == "Hello World" }
  end
end

Or:

describe First do
  describe "#to_s" do
    let(:result) { First.new.to_s }
    specify { result.should == "Hello World" }
  end
end

answered Mar 19, 2012 at 22:30

apneadiving

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