Modify Rails/Pluck to ignore map/collect when used inside blocks to prevent potential N+1 queries

[masato-bkn]

This PR modifies the Rails/Pluck cop to ignore map/collect when used inside blocks to prevent potential N+1 queries.

When the receiver's relation is not loaded and pluck is used inside an iteration, it can result in N+1 queries. This happens because pluck executes a database query on every iteration when the relation is not loaded.

Example

users = User.all
5.times do
  users.map { |user| user[:id] } # Only one query is executed
end

users = User.all
5.times do
  users.pluck(:id) # A query is executed on every iteration
end

users = User.all
users.load

5.times do
  users.pluck(:id) # Since the records are loaded, only one query is executed
end

As shown above, replacing map/collect with pluck in such cases can degrade performance due to the N+1 queries. To prevent this, it is better to ignore map/collect when used inside an iteration.

Challenge

Since Ruby has numerous methods for performing iterations, it is difficult to list all of them. Therefore, this PR assumes that map/collect used inside blocks are likely part of an iteration and ignores offenses.

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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.