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Add collection querying rule #962

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31 changes: 31 additions & 0 deletions README.adoc
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Expand Up @@ -4690,6 +4690,37 @@ ary[..42]
ary[0..42]
----

=== Collection querying [[collection-querying]]

When possible, use https://docs.ruby-lang.org/en/master/Enumerable.html#module-Enumerable-label-Methods+for+Querying[predicate methods from `Enumerable`] rather than expressions with `#count`, `#length` or `#size`.

Querying methods express the intention more clearly and are more performant in some cases. For example, `articles.any?(&:published?)` is more readable than `articles.count(&:published?) > 0` and also more performant because `#any?` stops execution as soon as the first published article is found, while `#count` traverses the whole collection.

[source,ruby]
----
# bad
array.count > 0
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Btw, there's also length to consider. ;-)

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Yep, updated. Note just that I wouldn't include length/size in the cop because it would yield too many false positives since Ruby has core classes which implement those methods, but aren't Enumerable (e.g. Integer, String, File, etc.).

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Yeah, I figured as much although probably this should be also mentioned in the cop's docs as well, and we can also provide "conservative/aggressive" modes there in case someone wants to do a broader search in their codebase (e.g. on a one off basis). Anyways, that's not relevant to the PR here.

array.length > 0
array.size > 0

array.count(&:something).positive?

array.count(&:something) == 0

array.count(&:something) == 1

# good
array.any?
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Can we exclude this?
Would it be possible to write a separate guideline for this?
If we harvest real-world-ruby-apps repos, what usages of count will there be?

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Why do you think this should be separated?

For methods like count you should also keep in mind that it's a different method in Enumerable and ActiveRelation and it will probably be hard for us to tell them apart with a simple search.

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Indeed, but if it’s not a list of repos of Rails apps like real-world-ruby-apps, chances are higher that it’s an Enumerable.

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Using any? instead of count may work, but is a time bomb, which will blow up when falsey values start appearing in the data.

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I think we just need a couple of examples illustrating the behavior with such values, perhaps in a sidebar/callout.


array.any?(&:something)

array.none?(&:something)

array.one?(&:something)
----

NOTE: Predicate methods without arguments aren't applicable when collection includes falsy values (e.g. `[nil, false].any?` evaluates to `false`).

== Numbers

=== Underscores in Numerics [[underscores-in-numerics]]
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