From d3ac3327731f2e1c2f4ab648a1bece91238758c6 Mon Sep 17 00:00:00 2001
From: Sam Bostock <sam.bostock@shopify.com>
Date: Mon, 21 Sep 2020 13:17:34 -0400
Subject: [PATCH] Add rule against memoizing possibly falsely values

"Avoid using `||=` to initialize boolean variables" provides a good
explanation about `false` not being safe to use with `||=`, but cases
where the value can be `nil` (e.g. not found) should also be considered.
---
 README.md | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/README.md b/README.md
index 00055a06..801b76e9 100755
--- a/README.md
+++ b/README.md
@@ -373,6 +373,21 @@ documentation](https://docs.rubocop.org/rubocop/configuration.html#inheriting-co
   @enabled = true unless defined?(@enabled)
   ~~~
 
+* Avoid using `||=` to memoize possibly falsey (`false` or `nil`) values.
+
+  ~~~ ruby
+  # bad - will repeat lookup each time if it returns nil
+  def existing_record
+    @existing_record ||= expensive_lookup_that_might_return_nil
+  end
+
+  # good
+  def existing_record
+    return @existing_record if defined?(@existing_record)
+    @existing_record = expensive_lookup_that_might_return_nil
+  end
+  ~~~
+
 * Avoid spaces between a method name and the opening parenthesis.
 
 * Prefer the lambda literal syntax over `lambda`.