group-by-with-default.md

group_by with default

Here's some code that made me do a double take.

something = items[nil]

It's that [nil]. What's going on here?

It turns out that items is the return of a call to Ruby's group_by. group_by groups by whatever the block evaluates to, including nil.

[
  { shipping: 'fedex', id: 1 },
  { shipping: 'fedex', id: 2 },
  { id: 3 }
].group_by { |item| item[:shipping] }

=> {
  'fedex' => [{ shipping: 'fedex', id: 1 }, { shipping: 'fedex', id: 2 }],
  nil => [{ id: 3 }]
}

Hence the key nil.

Here's another technique that I think is friendlier to the programmer:

[
  { shipping: 'fedex', id: 1 },
  { shipping: 'fedex', id: 2 },
  { id: 3 }
].group_by { |item| item[:shipping] || 'no_carrier' }

=> {
  'fedex' => [{ shipping: 'fedex', id: 1 }, { shipping: 'fedex', id: 2 }],
  'no_carrier' => [{ id: 3 }]
}

And if we rename our variable item to shipment_groups, then items[nil] becomes shipment_groups['no_carrier']. A clear readability upgrade.