Add suggestions for checking if collection is empty

[fatkodima]

Reference rubocop/rubocop-rails#1529

[andyw8]

Suggested change

	When checking if the unloaded Active Record collection is empty, prefer `any?`/`empty?` over `present?`/`blank?`. The former executes a simple `EXISTS`-like query while the latter loads the whole collection to determine it's size.
	When checking if the unloaded Active Record collection is empty, prefer `any?`/`empty?` over `present?`/`blank?`. The former executes a simple `EXISTS`-like query while the latter loads the whole collection to determine its size.

[pirj]

This is even more descriptive.
user.articles.any? is used just as frequently as users.where(...).any?, and in the former case, any? behavior is not equivalent to exists?, and is not behaving similarly to empty?.

[pirj]

I'm not convinced that preloading the list of records implicitly with present? is a practice we should recommend to everyone.

[pirj]

This is generic. The guideline is specific to "not loaded collection".
It is hard to know if it's loaded or not if it e.g. comes as an argument to a method, or elsewhere:

def highlight?
  recent_articles.any?
end

[pirj]

What if there are hundreds of thousands of those records?

[pirj]

I am referring to the "used" wording above.

Even though the collection will be used a few lines below, maybe only a fraction of it will be actually used with a batching iterator like `users.find_each { ...?

Or it will be further queried upon e.g. users.active? Or users.last(3).

Or, it will be used in a sub-query, and it doesn't meet the purpose to load them all e.g. Articles.recent.joins(:author).merge(users).

[pirj]

https://github.com/rubocop/rails-style-guide/blob/9a1323aa4c4ccfac10ccb9b77b8817e232f8338b/README.adoc#find-each

[fatkodima]

@pirj If you have suggestions for better wording/examples, I think it is better to force push into the PR. I used simple examples (not caring about each vs find_each etc) just for example purposes.