Fix unscoping bug in preload and includes

[k0kubun]

This PR fixes #11036.

Currently unscoped block does not work for includes and preload.
I changed ActiveRecord::Associations::Preloader to use all instead of default_scoped.
And ignore :preload and :includes scope to pass #5667.

[rafaelfranca]

Tests are broken with this patch.

[egilburg]

Seems it broke cases designed to cover #5667. The way I understand that issue, though, is that it used unscoped as a means to avoid infinite recursion bug, rather than as its own design decision.

[k0kubun]

Ignoring scoping closes a chance to unscope.
At first, I couldn't understand why test_preload_ignores_the_scoping exists.

The way I understand that issue, though, is that it used unscoped as a means to avoid infinite recursion bug, rather than as its own design decision.

I see. I understand test_preload_ignores_the_scoping is not a design but a way to fix test_scoping_with_a_circular_preload.

I'll find another way to fix circular preload without ignoring the scoping.

[k0kubun]

I fixed broken tests. 93be88e
I changed ActiveRecord::Associations::Preloader::Association#build_scope not to ignore scoping except preload and includes scope.

In this line, the klass's preload/includes values are already preloaded/included.
So I unscoped only preload/includes scoping. That fixes #5667 without ignoring all scoping.

[egilburg]

I'd combine both changelogs into one.

Unscope preload and includes in unscoped block. Preload and includes scope is still ignored for #5667.

Fixes #11036.

*Takashi Kokubun*

[egilburg]

Does this change deserve an independent spec? It's behavior that could be reused for other purposes.

[k0kubun]

preload changes preload_values.
:preload is in not SINGLE_VALUE_METHODS but MULTI_VALUE_METHODS.
Thus symbol_unscoping changes preload_values and it works well.

I think the feature of unscoping preload itself is reasonable because currently unscoping includes is supported.
I'll add some tests which assert it works.

[k0kubun]

I added the test ensuring that unscoping preload works. 6c19f24
Then I squashed all commits into 1145c48.

[k0kubun]

I'd combine both changelogs into one.

Thank you for a nice suggestion.
I squashed commits and combined changelogs. 69bb040

[k0kubun]

#11036 is resolved by #17360