Class: RuboCop::Cop::Vicenzo::RSpec::SubjectIsMethodResult

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

Overview

The subject of an example group that describes a method must be the object under test, not what the method returned.

A specification is read as a sentence. In "the cat caught the ball", the subject is the cat, not the ball. Storing the return value in the subject inverts that: the object the example is about disappears from the text, the subject's name starts to lie (a subject(:cat) holding a Symbol), and every scenario that would need the same object in a different state has to redefine the whole call.

An example group names a method when its description reads #instance_method, .class_method, or the bare method name. The cop then looks at the subjects declared under it - including the ones nested in a context - and reports the ones whose value comes from calling that method.

Some layers legitimately specify a single entry point, where the result is the subject (a service object always invoked through call, for instance). AllowedMethods exists for those and ships empty: the convention belongs to the project that adopts it, not to this gem. Declaring it there - or excluding a whole directory through the standard Exclude - keeps the exemption visible and reviewable in .rubocop.yml.

An inline disable directive is not that: it is an exemption nobody reviewed. Dropping the # from the description is not either - the cop reads bare descriptions too, and the only thing lost is the reader's clue that a method is being specified there.

Examples:

# bad - the subject is the ball
describe '#catch' do
  subject(:catch) { Cat.new(name: 'Bixano').catch(object: Ball.new) }

  it { is_expected.to eq(:success) }
end

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

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

a description without a prefix names a method too

# bad - dropping the '#' silences nothing and costs the reader the
# one clue that a method is being specified here
describe 'catch' do
  subject(:catch) { Cat.new(name: 'Bixano').catch(object: Ball.new) }
end

AllowedMethods: ['call']

# good - a layer whose convention is a single entry point. Declared in
# .rubocop.yml, the exemption stays visible and reviewable, which an
# inline disable directive never is
describe '.call' do
  subject(:result) { described_class.(object: Ball.new) }
end

Constant Summary collapse

MSG = 
'Subject holds what `%<method>s` returned, not the object under test. ' \
'Make the subject the object that receives `%<method>s`, and call it inside the example.'

Constants included from DescribedMethod

DescribedMethod::BARE_METHOD_DESCRIPTION, DescribedMethod::PREFIXED_METHOD_DESCRIPTION

Instance Method Summary collapse

Instance Method Details

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



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# File 'lib/rubocop/cop/vicenzo/rspec/subject_is_method_result.rb', line 68

def on_block(node)
  return unless subject?(node)

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

  invocation = call_in_chain(node.body) { |call| call.method?(method_name) }
  return unless invocation

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