Standardize error message of association matcher

[vsppedro]

I noticed that the association matcher was returning a slightly different message between the tests below:

shoulda-matchers/spec/unit/shoulda/matchers/active_record/association_matcher_spec.rb

Lines 1451 to 1454 in 9763c17

	expected_message = 'Expected Person to have a has_one association called detail ' \
	'(PersonDetail does not have a ["company_id", "person_detail_id"] foreign key.)'

shoulda-matchers/spec/unit/shoulda/matchers/active_record/association_matcher_spec.rb

Lines 1436 to 1439 in 9763c17

	expected_message = 'Expected Person to have a has_one association called detail ' \
	'(PersonDetail does not have a [:foo_id, :bar_id] foreign key.)'

Pay attention to the foreign_keys, their types are different. This PR standardized the error message - in both cases the foreign_keys will be symbols. Besides that, these modifications fix one of the four tests that are failing with Rails 6.1.

[mcmire]

I just had two comments. This error message is not the greatest anyways, IMO I don't know if it always points users to what the problem actually is or what the solution is, but that is not your fault and seems out of scope for your change here, so I'm okay with letting it slide.

Out of curiosity, which test failed on Rails 6.1 and why?

[vsppedro]

I'm opening this PR, #1418, just to show the errors that are happening with Rails 6.1.

One of them is:

3) Shoulda::Matchers::ActiveRecord::AssociationMatcher have_one rejects an association with a valid :class_name and :foreign_key option (CPK), but no columns
     Failure/Error:
       expect do
         have_one(:detail).class_name('PersonDetail').
           with_foreign_key(%i[company_id person_detail_id])
       end.not_to match_against(Person.new).and_fail_with(expected_message)
     
       Expected the positive version of the matcher not to match and for the failure
       message to be:
     
       ------------------------------------------------------------------------
       Expected Person to have a has_one association called detail (PersonDetail does not have a ["company_id", "person_detail_id"] foreign key.)
       ------------------------------------------------------------------------
     
       However, it was:
     
       ------------------------------------------------------------------------
       Expected Person to have a has_one association called detail (PersonDetail does not have a [:company_id, :person_detail_id] foreign key.)

Before Rails 6.1 this lines was returning an array of symbols:

shoulda-matchers/lib/shoulda/matchers/active_record/association_matcher.rb

Line 1448 in 9763c17

foreign_key_reflection.foreign_key

But now is returning a string. Example: "[:company_id, :person_detail_id]". The logic on the association matcher checks if this is an array:

shoulda-matchers/lib/shoulda/matchers/active_record/association_matcher.rb

Lines 1453 to 1458 in 9763c17

	if key.is_a?(Array)
	key.map(&:to_s)
	else
	key.to_s
	end

This logic was supposed to update that value to "["company_id", "person_detail_id"], but it's not working because it's not returning an array anymore.

I think this was the commit that made this impact: rails/rails@c0750fe

Standardizing the error message to be symbols solves this error. What do you think?

[mcmire]

@vsppedro As long as we don't change what the developer is passing to with_foreign_key when printing out the failure message, this makes sense to me. I say that because if the developer passes ["company_id", "person_detail_id"] but the failure message seems to indicate they passed [:company_id, :person_detail_id] then they might get confused (or, it would give us confusing information if we attempted to help them). It doesn't seem like that that's what you're doing here, though, so I'm good with what you've done so far here.

[vsppedro]

I'll go back to this as soon as I finished dropping rails 4.2, 5.0 and 5.1.

[vsppedro]

Done!

[vsppedro]

I decided to merge this PR into #1418. I think it will be simpler to work with just one branch.