ActiveRecord group not returning hash with arrays of objects for values

Question

I know this has been asked and answered a lot. I have two tables Foo and Bar.

class Foo < ApplicationRecord
  belongs_to :bar
  ...

Foo has attributes of id and name and bar_id

and

class Bar < ApplicationRecord
  has_many :foos
  ...

Bar has the attributes, id and name .

When I simply try Foo.group(:bar_id) I get #<Foo::ActiveRecord_Relation:0x3fdeac1cc274>

With Foo.group(:bar_id).count I get {5=>2, 1=>2} the keys being the bar_id and the values the count of how many have that id.

What I'm trying to do is, group Foo on Bar#name with an array of Foo s as the values.

{
 'name1' => [#<Foo:0x00007fbd5894f698 id:1, name: 'thing'...}, ...],
 'name2' => [#<Foo:0x00017fbd5894f698 id:5, name: 'thing'...}, ...],
 ...
}

With Foo.joins(:bar).group('bars.name').count I am able to return {"name1"=>2, "name2"=>2} But not an array of the Foo models. I know it's because of the count. But without the count it simply returns #<Foo::ActiveRecord_Relation:0x3fdeac1cc274>

I see a lot of suggestions using Enumerable#group_by . I don't want to use an enumerable as I'm using ActiveRecord and as the records increase, it will drastically slow down the look up.

Answer 1

I've noticed that you're using PostgreSQL. Why not then use the json aggregation functions. It a bit differs from your desired result, but still contains the same information:

Bar
  .joins(:foos)
  .group("bars.name")
  .pluck("bars.name", "json_agg(json_build_object('id', foos.id, 'name', foos.name))")
  .to_h

The result is going to be:

{
 'name1' => [{id:1, name: 'thing'}, ...],
 'name2' => [{id:5, name: 'thing'}, ...],
 ...
}