What is the expected syntax for checking exception messages in MiniTest's assert_raises/must_raise?

What is the expected syntax for checking exception messages in MiniTest's assert_raises/must_raise?

I'm trying to make an assertion something like the following, where "Foo" is the expected error message:

proc { bar.do_it }.must_raise RuntimeError.new("Foo")

You can use the assert_raises assertion, or the must_raise expectation.

it "must raise" do
  assert_raises RuntimeError do 
    bar.do_it
  end
  ->     { bar.do_it }.must_raise RuntimeError
  lambda { bar.do_it }.must_raise RuntimeError
  proc   { bar.do_it }.must_raise RuntimeError
end

If you need to test something on the error object, you can get it from the assertion or expectation like so:

describe "testing the error object" do
  it "as an assertion" do
    err = assert_raises RuntimeError { bar.do_it }
    assert_match /Foo/, err.message
  end

  it "as an exception" do
    err = ->{ bar.do_it }.must_raise RuntimeError
    err.message.must_match /Foo/
  end
end

To assert exception:

assert_raises FooError do
  bar.do_it
end

To assert exception message:

As per API doc, assert_raises returns the exception matched so you can check the message, attributes, etc.

exception = assert_raises FooError do
  bar.do_it
end
assert_equal('Foo', exception.message)

Minitest does not provide (yet) you a way to check the actual exception message. But you could add a helper method that does it and extend ActiveSupport::TestCase class to use everywhere in your rails test suite, e.g.: in test_helper.rb

class ActiveSupport::TestCase
  def assert_raises_with_message(exception, msg, &block)
    block.call
  rescue exception => e
    assert_match msg, e.message
  else
    raise "Expected to raise #{exception} w/ message #{msg}, none raised"
  end
end

and use it in your tests like:

assert_raises_with_message RuntimeError, 'Foo' do
  code_that_raises_RuntimeError_with_Foo_message
end

To add some more recent developments, there have been some discussions on adding assert_raises_with_message to minitest in the past without much luck.

Currently, there is a promising pull request waiting to be merged. If and when it gets merged, we will be able to use assert_raises_with_message without having to define it ourselves.

In the meanwhile, there is this handy little gem named minitest-bonus-assertions which defines exactly that method, along with few others, so that you can use it out of the box. See the docs for more information.