How do I assert on an error message in Ruby unit testing?

Question

In a Ruby on Rails project, I want to assert on the exception message in my code to make sure it fails for the right reason and mentions important details.

Here is a function that always raises:

class Quiz < ApplicationRecord
  def self.oops
    raise ArgumentError.new("go away")
  end
end

The test:

require "test_helper"

class QuizTest < ActiveSupport::TestCase
  test "error message" do
    assert_raises(ArgumentError, match: /whatever/) do
      Quiz.oops()
    end
  end
end

When I run bin/rake test, this passes, but I expected it to fail cause the actual error message doesn't match the match in assert_raises.

How can I capture the error message and assert against it?

Answer 1

I think you can use a little different approach (capture the error message using a block and then assert on the message content):

test "error message" do
  exception = assert_raises(ArgumentError) do
    Quiz.oops()
  end
  assert_match(/whatever/, exception.message)
end