Add Use Slice To Reorder Hash Keys as a Ruby TIL

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

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+# 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.