Group Rails query into arrays by attribute

Question

Suppose I have these rows: (represented as JSON):

[
  { name: "Bob", age: 10 },
  { name: "Carl", age: 15 },
  { name: "Alice", age: 10 },
  { name: "Derek", age: 20 }
]

How can I, in Rails, group these by age? For example, I want something like this:

[
  { age: 10, objects: [
    { name: "Bob", age: 10 },
    { name: "Alice", age: 10 }
  ] },
  { age: 15, objects: [
    { name: "Carl", age: 15 }
  ] },
  { age: 20, objects: [
    { name: "Derek", age: 20 }
  ] },
]

score 27 · Accepted Answer · answered Sep 18 '13 at 01:14

27

Got it!

People.all.group_by(&:age)

answered Sep 18 '13 at 01:14

gberger

2,813
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What is the ampersand for? – picheto Feb 13 '20 at 00:28
1

it's a shortcut for turning the `:age` into a proc, see here: https://stackoverflow.com/questions/1217088/what-does-mapname-mean-in-ruby – Michael Evans Feb 26 '20 at 17:25

score 10 · Answer 2 · edited Jun 24 '17 at 01:21

10

If you're actually dealing with JSON:

people.group_by{|p| p['age'] }

If you're dealing with ActiveRecord models:

People.group('id, age')

Here there's additional documentation on grouping with ActiveRecord.

edited Jun 24 '17 at 01:21

Sebastián Palma

32,692
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answered Sep 18 '13 at 01:16

agmin

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27

I like the one with the block! Unfortunately I named an association the same way as a column in my PG view and Rails was trying to do some "unexpected" magic with that. Luckily I was able to do akin to `people.group_by{|p| p.read_attribute(:age) }`. – mlt Jan 23 '20 at 19:27

Group Rails query into arrays by attribute

2 Answers2