Reason #101 • April 11th, 2026

ActiveSupport: Enumerable#index_by

If Enumerable#group_by is for "many values per key", then index_by is for "exactly one value per key". Think of it as applying a unique index to an enumerable:

Ruby
people = [
  { id: 101, email: "[email protected]" },
  { id: 102, email: "[email protected]" },
  { id: 103, email: "[email protected]" }
]

people_by_id = people.index_by { it[:id] }
# => {
#   101 => { id: 101, email: "[email protected]" },
#   102 => { id: 102, email: "[email protected]" },
#   103 => { id: 103, email: "[email protected]" }
# }

people_by_id[102]
# => { id: 102, email: "[email protected]" }
JavaScript
const people = [
  { id: 101, email: "[email protected]" },
  { id: 102, email: "[email protected]" },
  { id: 103, email: "[email protected]" }
];

const peopleById = Object.fromEntries(
  people.map((person) => [person.id, person])
);
// => {
//   101: { id: 101, email: "[email protected]" },
//   102: { id: 102, email: "[email protected]" },
//   103: { id: 103, email: "[email protected]" }
// }

peopleById[102];
// => { id: 102, email: "[email protected]" }

Note: If two elements produce the same key, the last one wins.

History

Enumerable#index_by was added to ActiveSupport in Rails 1.2.0, released in 2007.

Reason #102 ?