Improve multiple belongs_to hack

[leio10]

This PR slightly modifies polymorphic routes generation. When mapping the route parameters, instead of checking each class of the given parameters against the parent class, it does it against the configured belongs_to resource class. This has sense because if this check is successful it won't use the parent but the configured belongs_to resource.

As explained in #221, when you define several belongs_to for a resource:

ActiveAdmin.register Movie do
  belongs_to :provider, optional: true
  belongs_to :actor, optional: true
end

And then add the missing routes:

namespace :admin do
    resources :movies
    resources :providers do
        resources :movies
    end
    resources :actors do
        resources :movies
    end
end

Everything seems to work well, but when browsing the movies for the provider 123, some links are generated as /actors/123/movies instead of /providers/123/movies.

This change fixes this issue and keeps the same behavior when having only one belongs_to declaration.

Also, I've added some tests to avoid regression, even when multiple belongs_to is still not supported, since it can be useful for many people, and could be a little step further toward supporting this 😃 .

[deivid-rodriguez]

Hi @leio10! Thanks, this makes sense to me!

Can you add a change log entry following the existing format and squash everything into a single commit?

[leio10]

Hi @deivid-rodriguez, thanks for the review!

I did what you asked for, but now seems that JS linting is failing for some strange reason 🤕

[leio10]

Ok, @deivid-rodriguez, it was my mistake. I got confused with the Gherkin error, which is actually ignored. The real issue was that my changelog entry wasn't pointing to the right URL. I'm impressed that you have a test for that. 😍 🙌

[deivid-rodriguez]

Thanks @leio10! Still gherkin-lint failing is unexpected, trying to fix it in #5941.

[deivid-rodriguez]

💜

[javierjulio]

@deivid-rodriguez I see you pushed the latest master changes to get this to pass. Thanks again! Just wanted to confirm this was ready in case you may have needed to do something else?

[deivid-rodriguez]

Yeah, I fixed changelog conflicts through github's UI in order to not bother @leio10 anymore. It's ready (I would "Squash and merge" to get rid of the merge commit).

[javierjulio]

Oh don't you worry, I'll be getting rid of that merge commit. 😁 Thanks again!

[javierjulio]

Thanks you two! ❤️