Class: RuboCop::Cop::Vicenzo::RSpec::LetIsMethodResult

Inherits:
RSpec::Base
  • Object
show all
Includes:
DescribedMethod, PremiseTracking
Defined in:
lib/rubocop/cop/vicenzo/rspec/let_is_method_result.rb

Overview

A let under an example group that describes a method must not hold what calling that method on the subject returned.

This is the same inversion Vicenzo/RSpec/SubjectIsMethodResult reports, moved one definition away: the subject stays the object under test, and the result is parked in a let that the expectation then asserts on. What the example says out loud becomes a name, and the action - the subject receiving the method - is nowhere in the example that is supposed to describe it.

The fix is not to rename the let or to move it: it is to undo it. Even when the let is read inside an expect, what it holds belongs in the expectation itself, where the sentence reads whole - the cat catches the ball, and that is what is asserted.

Only calls on the subject count, which is what separates the result from the setup: a let calling the described method on some other object is building a premise, not the outcome the example is about. AllowedMethods is shared with the sibling cop and ships empty, for the layers whose convention is a single entry point.

Examples:

# bad - the expectation asserts on a name; the cat never catches anything in the example
describe '#catch' do
  subject(:cat) { Cat.new(name: 'Bixano') }

  let(:catch) { cat.catch(object: Ball.new) }

  it { expect(catch).to eq(:success) }
end

# good - the action is in the expectation
describe '#catch' do
  subject(:cat) { Cat.new(name: 'Bixano') }

  it { expect(cat.catch(object: Ball.new)).to eq(:success) }
end

a call on another object is a premise, not the result

# good - the described method builds the state the example starts from
describe '#catch' do
  subject(:cat) { Cat.new(name: 'Bixano') }

  let(:taken_ball) { other_cat.catch(object: Ball.new) }
end

Constant Summary collapse

MSG = 
'Let `:%<name>s` holds what `%<method>s` returned. Undo it and call `%<method>s` on the ' \
'subject inside the expectation.'

Constants included from PremiseTracking

PremiseTracking::SETUP_HOOKS

Constants included from DescribedMethod

DescribedMethod::BARE_METHOD_DESCRIPTION, DescribedMethod::PREFIXED_METHOD_DESCRIPTION

Instance Method Summary collapse

Methods included from PremiseTracking

#premise_name, #root_receiver_name

Instance Method Details

#on_block(node) ⇒ Object Also known as: on_numblock, on_itblock 



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# File 'lib/rubocop/cop/vicenzo/rspec/let_is_method_result.rb', line 58

def on_block(node)
  return unless let?(node) || let_it_be?(node)

  method_name = described_method_name(node)
  return if method_name.nil? || allowed_methods.include?(method_name)

  invocation = invocation_on_subject(node, method_name)
  return unless invocation

  add_offense(invocation, message: format(MSG, name: premise_name(node), method: method_name))
end