Custom assert message for multiple expect statements within match

Question

I have written a custom match method in Rspec that matches an object against a hash. What I am trying to do is set custom failure messages for each line of the expect.

describe "/cars" do
  car = FactoryGirl.create(:car, name: 'Alpha')
  describe car do
    it "displays single items" do
      get cars_path
      parsed_response = JSON.parse(response.body)
      record_hash = parsed_response['cars'][0]
      is_expected.to be_a_car_match_of(record_hash)
    end
  end
end

RSpec::Matchers.define :be_a_car_match_of do |hash|
  match do |car|
    expect(car.id).to    eq(hash['id'])
    expect(car.name).to  eq(hash['name'])
  end
  failure_message do |car|
    "expected that #{car} would be a match of #{hash}"
  end
end

So what I would like is to have something like the following:

RSpec::Matchers.define :be_a_car_match_of do |hash|
  match do |car|
    expect(car.id).to    eq(hash['id'])    'ids did not match'
    expect(car.name).to  eq(hash['name'])  'names did not match'
  end
end

This would print out a much clearer error message.

I was initially doing this in mini-test but for a variety of reasons (outside of my control) needed to change it to rspec. The code I had in mini-test was:

def assert_car(car, hash)
    assert_equal car.id,    hash['id'],   "ids did not match"
    assert_equal car.name,  hash['name'], "names did not match"
end

This is what I am trying to replicate.

Here is another example that requires less setup:

require 'rspec/expectations'

RSpec::Matchers.define :be_testing do |expected|
  match do |actual|
    expect(5).to eq(5)
    expect(4).to eq(5)
  end
  failure_message do
    "FAIL"
  end
end

describe 'something' do
  it 'something else' do
    expect("expected").to be_testing('actual')
  end
end

When this example is run, "FAIL" is printed out. On the other hand if I had:

describe 'something' do
  it 'something else' do
    expect(4).to eq(5)
  end
end

I would get the following error message:

expected: 5
     got: 4

This is what I want. I want to know what part of the custom matcher failed.

Answer 1

You could call the low-level matches? method instead of expect. Something like this:

require 'rspec/expectations'

RSpec::Matchers.define :be_a_positive_integer do

  m1, m2 = nil, nil           # matchers
  r1, r2 = false, false       # results

  match do |actual|
    m1 = be_a Integer         # this returns matcher instances
    m2 = be > 0

    r1 = m1.matches?(actual)  # evaluate matchers
    r2 = m2.matches?(actual)

    r1 && r2                  # true if both are true
  end

  failure_message do |actual| # collect error messages from matchers
    messages = []
    messages << m1.failure_message unless r1
    messages << m2.failure_message unless r2
    messages.join("\n")
  end

end

describe -1 do
  it { is_expected.to be_a_positive_integer }
end

describe 1.0 do
  it { is_expected.to be_a_positive_integer }
end

describe -1.0 do
  it { is_expected.to be_a_positive_integer }
end

Output:

Failures:

  1) -1 should be a positive integer
     Failure/Error: it { is_expected.to be_a_positive_integer }
       expected: > 0
            got:   -1
     # ./ruby_spec.rb:24:in `block (2 levels) in <top (required)>'

  2) 1.0 should be a positive integer
     Failure/Error: it { is_expected.to be_a_positive_integer }
       expected 1.0 to be a kind of Integer
     # ./ruby_spec.rb:28:in `block (2 levels) in <top (required)>'

  3) -1.0 should be a positive integer
     Failure/Error: it { is_expected.to be_a_positive_integer }
       expected -1.0 to be a kind of Integer
       expected: > 0
            got:   -1.0
     # ./ruby_spec.rb:32:in `block (2 levels) in <top (required)

Answer 2

I think aggregate_failures is what you are looking for :

It wraps a set of expectations with a block. Within the block, expectation failures will not immediately abort like normal; instead, the failures will be aggregated into a single exception that is raised at the end of the block, allowing you to see all expectations that failed.

See : https://relishapp.com/rspec/rspec-expectations/docs/aggregating-failures