Factor out deep_merge into AS::DeepMergeable

[jonathanhefner]

The ActiveSupport::DeepMergeable module allows a class to provide deep_merge and deep_merge! methods simply by implementing a merge!(other, &block) method. Values will be deep merged only when they are compatible, according to deep_merge?. By default, that only includes instances of the same class or its subclasses. A class may override deep_merge? to further restrict or expand the domain of deep mergeable values.

This does introduce a small change in behavior. Previously, Hash#deep_merge would only deep merge Hash instances. Now, deep_merge will deep merge any DeepMergeable instances that are compatible with each other.

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This was inspired by #45369 (comment). For now, I've marked ActiveSupport::DeepMergeable as :nodoc:, but I can remove that if we want to make it publicly available.

[seanpdoyle]

#45369 has been stale for some time now, so I've rebased it and pushed up the changes.

How else can I help to move these changes along as well?

[morgoth]

hi @jonathanhefner - since it's changing the current behavior, shouldn't it have a changelog entry?

[jonathanhefner]

since it's changing the current behavior, shouldn't it have a changelog entry?

I'm not sure if that's necessary... Currently, ActiveSupport::DeepMergeable isn't a public API, and the only two classes that include it are Hash and ActionController::Parameters. So the only thing that has changed is something like { foo: params }.deep_merge({ foo: other_params }). (The value of :foo will now be params.deep_merge(other_params) instead of other_params.)

I think the changelog entry from #45369 might be sufficient. But I can add another entry if you feel it would be helpful.