Introduce reload_<association> reader for singular associations.

[senny]

Summary

This patch brings back the functionality of passing true to the
association proxy. The behavior was deprecated with #20888 and scheduled
for removal in Rails 5.1.

The deprecation mentioned that instead of Article.category(true) one
should use article#reload.category. Unfortunately the alternative does
not expose the same behavior as passing true to the reader
did. Specifically reloading the parent record throws unsaved changes and
other caches away. Passing true only affected the association.

This is problematic and there is no easy workaround. I propose to bring
back the old functionality by introducing this new reader method for
singular associations.

Implications

We started to issue deprecation warnings for this since 5.0.0. If this is merged, I think we should backport the patch to 5-0-stable and update the deprecation warning accordingly.

[senny]

@dhh we discussed this some time ago. If possible I'd like to bring this back to 5.0.x and update the deprecation warnings. The current suggestion to change to article.reload.category can cause bugs that are hard to track down.

/cc @matthewd

[dhh]

I thought we had agreed on using #reload_category or another dynamic method? I don't like using a mystery boolean like that. It's a terrible API (even if I originally designed it!).

[kaspth]

@dhh that is what Yves is proposing in this PR. 😊

[dhh]

Ha! I should have read the diff, not the description. Silly me. 👍

[bogdan]

The force reloading of the association reader was deprecated in #20888.

Sounds very sad.

[kaspth]

I see the deprecation warning in the reader method still says passing true is deprecated, but recommends calling reload on the parent object.

We should probably change that wording now :)

[senny]

@kaspth I'm on it. I guess for master the message will just be removed along with the possibility to pass true. Or am I mistaken? My backport to 5-0-stable will change the wording.

[kaspth]

I guess for master the message will just be removed along with the possibility to pass true.

👍. But how will 5.0 users who have replaced the category(true) instances with reload.category then see our changed recommendation? A mention in the 5.1 release notes?

My backport to 5-0-stable will change the wording.

👍. We should probably update the release notes as well.

[senny]

Backported with ecb394a

[matthewd]

Anyone successfully using the old recommendation is fine. It's only insufficient for a narrow use case.

(Do we want to recommend both? Or is that just confusing?)

[senny]

@matthewd I don't think it's necessary. Honestly now that we have reload_article I would recommend everyone who was passing true to use that. Reloading the parent has many more side effects and is simply not needed when all you want is the latest version of the association.