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

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 Hash#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:

> 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.