Add new MigrationClassName cop

[j-miyake]

This cop makes sure that each migration file defines a migration class whose name matches the file name.
(i.e. 20220224111111_create_users.rb should define CreateUsers class.)

The migration will fail when you rename the migration class and forget to rename the file name appropriately. Unfortunately, it's hard to detect this kind of bug with automated tests that use database tables restored from db/schema.rb (e.g. db:setup). The static inspection for the file names could help to prevent the bug.

---

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.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[andyw8]

I think this shouldn't mutate the word, i.e.:

word
  .gsub(/([A-Z\d]+)([A-Z][a-z])/, '\1_\2')
  .gsub(/([a-z\d])([A-Z])/, '\1_\2')
  .tr('-', '_')
  .downcase

[j-miyake]

That's true. Thanks!

[koic]

Can you move these logic in the on_class method and refactor to prevent the use of the instance variable @basename?
https://refactoring.com/catalog/inlineFunction.html

[koic]

Suggested change

	basename_without_timestamp = @basename.sub(/^[0-9]+_/, '')
	basename_without_timestamp = @basename.delete_prefix(/\A\d+_/, '')

[koic]

Oops! It was correctly as follows.

-          basename_without_timestamp = @basename.delete_prefix(/\A\d+_/, '')
+          basename_without_timestamp = @basename.sub(/\A\d+_/, '')

[koic]

It would be user-friendly to indicate the class name to replace.

Suggested change

	^^^^^^^^^ The class name does not match with the file name.
	^^^^^^^^^ Replace with `CreateUsers` that matches the file name.

[koic]

And I think this cop can provide autocorrect the class name.

[koic]

MigrationClassName seems to be more suitable than MigrationFileName.

Suggested change

	class MigrationFileName < Base
	class MigrationClassName < Base

[j-miyake]

That's absolutely right! Thanks!

[j-miyake]

I updated the title of this pull request as well.

[koic]

Maybe "corrected" is better than "preferred" . Because it ’s not a difference in style :-)

Suggested change

	MSG = 'Replace with `%<preferred_class_name>s` that matches the file name.'
	MSG = 'Replace with `%<corrected_class_name>s` that matches the file name.'

[koic]

Suggested change

	preferred_class_name = to_camelcase(basename_without_timestamp)
	message = format(MSG, preferred_class_name: preferred_class_name)
	corrected_class_name = to_camelcase(basename_without_timestamp)
	message = format(MSG, corrected_class_name: corrected_class_name)

[koic]

Suggested change

	add_offense(node.loc.name, message: message) do |corrector|
	corrector.replace(node.loc.name, preferred_class_name)
	end
	add_offense(node.identifier, message: message) do |corrector|
	corrector.replace(node.identifier, preferred_class_name)
	end

[koic]

Nice! Can you squash your commits into one?

[j-miyake]

Thanks for reviewing!

By the way, is there something like a wiki for writing better cops? I read source code in rubocop-ast repository to learn what node.identifier exactly does, but I would love to read the guide if it exists.

[koic]

Here is an introductory document, but I'm also learning by reading the source code.
https://docs.rubocop.org/rubocop/development.html