Rails 7 support

[flavorjones]

Alternative implementation to #267 that doesn't monkeypatch ActiveRecord.

This implementation subclasses ActiveRecord::Reflection::BelongsToReflection and overrides the compute_class method to allow ActiveHash associations.

I've also cherry-picked some relevant commits from #267.

[kbrock]

@flavorjones This looks great. A few thoughts:

• Will this require developers to mixin this belongsto module everywhere for this functionality? (I vaguely remember this module in the past)
• If so, we need to make sure this is a major version change with incompatibility note.
• Do we need to document this change so it is easy for developers to fix when upgrading to 7.0?

(as with everywhere in rubyland. thanks for the contribution)

[kbrock]

LGTM. just minor logistics questions

[flavorjones]

@kbrock Thanks for the review and for asking these questions.

This should "just work" for folks who have followed the README and are extending their AR classes with ActiveHash::Associations::ActiveRecordExtensions (see this section in the README).

However, it looks like not all the tests were following that advice but were still passing with Rails 6 (see the change in this PR to spec/active_hash/base_spec.rb), which seems suspicious and makes me believe there may be apps out there which are "working" but will break with Rails 7. Maybe I'm not fully understanding Active Hash and how it's supposed to be used? I could use some advice, honestly.

[flavorjones]

Looks like the ActiveRecordExtensions module was added in v0.9.6 (see 153c58e) but wasn't added to this test (probably because it wasn't failing).

[kbrock]

Thanks.
I thought I remembered this module from a number of years ago.

Ok, I change my view from breaking change to bug fix.

[flavorjones]

@kbrock I just saw now that I still had a "wip" message in that last commit, urk, sorry, I should have fixed that. Candidly, I did not expect that code to pass on every Rails version and assumed I'd be coming back to clean something up. 🤷

[kbrock]

@flavorjones ugh. very sorry that I did not see that. ping me to get that through