Why does changing the order of 'it' and 'subject' in RSpec change my test result? - ruby

Question

The class being tested qa.rb contains the code:
class QA
def initialize(bugs: 0)
#bugs = bugs
end
def speak
"Hello!"
end
def happy?
#bugs > 0
end
def debug
#bugs = 0
end
end
The RSpec file qa_spec.rb contains the code:
require 'rspec'
require_relative 'qa'
RSpec.describe QA do
describe '#happy?' do
context 'when bugs are more than 0' do
it 'returns true' do
subject { described_class.new(bugs: 1) }
expect(subject).to be_happy
end
end
end
end
The test fails when I run it, and gives me this error:
PS C:\Users\Jobla\repos\TDD> rspec qa_spec.rb
F
Failures:
1) QA#happy? when bugs are more than 0 returns true
Failure/Error: expect(subject).to be_happy
expected `#<QA:0x2e0d640 #bugs=0>.happy?` to return true, got false
# ./qa_spec.rb:9:in `block (4 levels) in <top (required)>'
Finished in 0.02999 seconds (files took 0.16995 seconds to load)
1 example, 1 failure
Failed examples:
rspec ./qa_spec.rb:7 # QA#happy? when bugs are more than 0 returns true
However, when I edit qa_spec.rb and I swap the it and subject lines, the test suddenly passes:
require 'rspec'
require_relative 'qa'
RSpec.describe QA do
describe '#happy?' do
context 'when bugs are more than 0' do
subject { described_class.new(bugs: 1) } #swapped with line below
it 'returns true' do #swapped with line above
expect(subject).to be_happy
end
end
end
end
Tests pass:
PS C:\Users\Jobla\repos\TDD> rspec qa_spec.rb
.
Finished in 0.01003 seconds (files took 0.17993 seconds to load)
1 example, 0 failures
Please could someone explain why does swapping the it and subject lines change the result of the test?

Answer 1

subject is designed to be set in context or describe block, but not in it.
If you do not set subject before it then subject would be set automatically by calling new without parameters on described_class. bugs will be set to default 0. After that, you call it with a block subject { described_class.new(bugs: 1) } inside it, it's the same as if you call described_class.new { described_class.new(bugs: 1) } because subject inside it is an instance of QA class.