Add new Rails/WhereMissing cop

[ydah]

Refs: https://rails.rubystyle.guide/#finding-missing-relationship-records

This cop is check for the use left_joins and where method
for finding missing relationship records.

# bad
Foo.left_joins(:foo).where(foos: { id: nil })

# good
Foo.where.missing(:foo)

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• [-] Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• 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.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[dvandersluis]

Looks like this will only catch code where the chaining is in the exact order prescribed here but order doesn’t matter in ActiveRecord.

None of these will be caught for example (and obviously there are many more forms):

Foo.where(foos: { id: nil }).left_joins(:foo)
# => Foo.where.missing(:foo)

Foo.left_joins(:foo).where(foos: { id: nil }, x: y)
# => Foo.where(x: y).where.missing(:foo)

Foo.left_joins(:foo).joins(:bar).where(foos: { id: nil })
# => Foo.joins(:bar).where.missing(:foo)

[koic]

@ydah Ping.

[ydah]

Oh, thank you so much. I'll work on it this week.

[ydah]

I updated this PR. Thank you so much.

[koic]

@ydah I think this cop can have autocorrection. Can you implement it?

[ydah]

I updated this PR. Auto-correction is now supported. Thank you so much.

[Darhazer]

I've run on a real code base and got

undefined method `value' for #<RuboCop::AST::HashNode:0x000000014fb49c90>
Did you mean?  values
rubocop-rails/lib/rubocop/cop/rails/where_missing.rb:51:in `same_relationship?'

It happens on

left_joins(foo: :bar).where(bars: { id: nil })

[ydah]

Oops, that case was leaking. Thank you so much.
I've updated this PR.

[Darhazer]

Sorry to be the bringer of bad news, but there are potential false positives in this cop.

Also with 2 offences within the same file, it managed to replace the left_joins with where.missing, but removed only one of the where(foos: {id: nil}) clauses.

[Darhazer]

Thank you. The cop no longer crashes or produces false positives, but there are false negatives. I guess we can't easily fix all of those though.

I left come final comments

[Darhazer]

🙇

I found no false positives, false negatives or incorrect corrections