Add cop for checking assingments to `ignored_columns`

[kkitadate]

This PR is a rebase of https://github.com/rubocop/rubocop-rails/pull/514 on rubocop:master. It is requested by https://github.com/rubocop/rubocop-rails/pull/514#issuecomment-1243272703.

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Before submitting the PR make sure the following are checked:

• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [x] Wrote good commit messages.
• [x] Commit message starts with [Fix #issue-number] (if the related issue exists).
• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [x] Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• [x] If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

The issue to be resolved conflicts with #761. We might need some time to figure out which one is better. Anyway, the next release is a bug fix, so we have time. cc @sikachu

[sikachu]

Oh, interesting. So this approach is to suggest that we always do += instead of = altogether.

I think this approach definitely makes the cop super clean, as you just detect for = instead of +=.

However, I find that it is a bit weird since if you look at just the model itself, you'll have to use += even for the first call. You have to have background knowledge of "Rails sets this variable to [], so that's why we can just use +=.

In our codebase, we never use += but we did have a few accidents of calling = a few times. Hence, I'm in favor of the = version since it's more clear without the need of the background knowledge.

[pirj]

figure out which one is better

I'd go with +=, keeping in mind inheritance and mixins.

[fsateler]

For reference, #761 also started as a cop checking for =. However, due to the impossibility of checking for mixins and inheritance, this cop might not flag invalid behavior, while it is always correct to use +=.

[sikachu]

Yeah, I'm retracting my #761. Let's use += since it works better for inheritance and it makes it much easier to auto-fix it. 👍

[koic]

Thank you for the opinions! Let us move on with this cop.

[kkitadate]

CI seems to have failed because rubocop/rubocop#11002 was merged 5 hours ago. https://github.com/rubocop/rubocop-rails/pull/798 will fix this failure.

[koic]

Thanks!