Convert Array of objects to Hash with a field as the key

Question

I have an Array of objects:

[
  #<User id: 1, name: "Kostas">,
  #<User id: 2, name: "Moufa">,
  ...
]

And I want to convert this into an Hash with the id as the keys and the objects as the values. Right now I do it like so but I know there is a better way:

users = User.all.reduce({}) do |hash, user|
  hash[user.id] = user
  hash
end

The expected output:

{
  1 => #<User id: 1, name: "Kostas">,
  2 => #<User id: 2, name: "Moufa">,
  ...
}

@SergioTulentsev, I was looking at Enumerable#group_by and it was almost what I'm looking for. I just thought there is a version of it the instead of building arrays for values, it is more agressive and keeps only one value. — Kostas
– Kostas, Commented Apr 2, 2013 at 10:10

tokland · Accepted Answer · 2018-11-21 11:40:13Z

80

users_by_id = User.all.map { |user| [user.id, user] }.to_h

If you are using Rails, ActiveSupport provides Enumerable#index_by:

users_by_id = User.all.index_by(&:id)

edited Nov 21, 2018 at 11:40

answered Apr 2, 2013 at 10:12

tokland

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2 Comments

Kostas Over a year ago

I personally prefer the mash way instead of the Hash[...] one. It read cleaner, more ruby-like.

Tron Over a year ago

I believe your Ruby >= 2.1 needs a slight correction, you want .to_h not .to_a e.g. users = User.all.map { |u| [u.id, u] }.to_h

Sergio Tulentsev · Accepted Answer · 2013-04-02 10:11:22Z

7

You'll get a slightly better code by using each_with_object instead of reduce.

users = User.all.each_with_object({}) do |user, hash|
  hash[user.id] = user
end

answered Apr 2, 2013 at 10:11

Sergio Tulentsev

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2 Comments

tokland Over a year ago

Are we going to go the inject/each_with_object/Hash/mash path again? :-) bugs.ruby-lang.org/issues/show/666

Sergio Tulentsev Over a year ago

@tokland: Yes, that was the first thought in my head when I saw your comment :)

Serge · Accepted Answer · 2024-09-16 18:17:24Z

3

You can simply using numbered parameter syntax of the Ruby 2.7+ :

users_by_id = User.all.to_h { [_1.id, _1] }

Note. In Ruby 3.4 + simply use it for the named parameter :

users_by_id = User.all.to_h { [it.id, it] }

edited Sep 16, 2024 at 18:17

Serge

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answered Oct 13, 2022 at 13:19

Jon Snow

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