Add Use Slice To Reorder Hash Keys as a Ruby TIL

README.md

@@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/).
 
 For a steady stream of TILs, [sign up for my newsletter](https://visualmode.kit.com/newsletter).
 
-_1828 TILs and counting..._
+_1829 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -1587,6 +1587,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Use A Case Statement As A Cond Statement](ruby/use-a-case-statement-as-a-cond-statement.md)
 - [Use dotenv In A Non-Rails Project](ruby/use-dotenv-in-a-non-rails-project.md)
 - [Use Rescue As Part Of Inline Statement](ruby/use-rescue-as-part-of-inline-statement.md)
+- [Use Slice To Reorder Hash Keys](ruby/use-slice-to-reorder-hash-keys.md)
 - [Use Tap For Better Test Data Setup](ruby/use-tap-for-better-test-data-setup.md)
 - [Using BCrypt To Create And Check Hashed Passwords](ruby/using-bcrypt-to-create-and-check-hashed-passwords.md)
 - [What To Do When You Don't Rescue](ruby/what-to-do-when-you-dont-rescue.md)

ruby/use-slice-to-reorder-hash-keys.md

@@ -0,0 +1,34 @@
+# Use Slice To Reorder Hash Keys
+
+I recently ran into some code that was building up a hash keyed by _category_.
+It was a series of chained method calls building and transforming to produce the
+hash. It then ended with `.slice(*CATEGORIES)`.
+
+That `#slice` call at the end was doing two things:
+
+1. It was removing any key-value pairs not in `CATEGORIES`
+2. It was reordering the hash keys based on the order they appeared in `CATEGORIES`
+
+It was this second behavior that surprised me.
+
+Let's look at a minimum example of this:
+
+```ruby
+> CATEGORIES = [:books, :clothes, :glassware]
+=> [:books, :clothes, :glassware]
+> items_by_category = { glassware: [1,2,3], media: [:a, :b, :c], books: [:e, 3, :g], clothes: [4, 5] }
+=> {glassware: [1, 2, 3], media: [:a, :b, :c], books: [:e, 3, :g], clothes: [4, 5]}
+> items_by_category.slice(*CATEGORIES)
+=> {books: [:e, 3, :g], clothes: [4, 5], glassware: [1, 2, 3]}
+```
+
+- I first define `CATEGORIES` which is an (ordered) array of symbols that
+represent the categories I care about.
+- Then I manufacture a hash of items by category. This is a bunch of dummy data,
+including extra categories, meant to demonstrate what `#slice` can do.
+- Lastly I call `.slice(*CATEGORIES)` on this hash which both pairs it down to
+that set of categories and reorders the hash so that the keys appears in the
+same order they do in the `CATEGORIES` array.
+
+This might be useful if I've defined `CATEGORIES` to be in a specific display
+order and I'm preparing this hash for consumption in some UI layer.