How to group-by and nest results in each group? - ruby-on-rails

Question

I tried to do this:
Things.order("name").group("category_name")
I was expecting the results to be something like this:
[
{
"category_name": "Cat1",
"things":
[
{ "name": "Cat1_Thing1" },
{ "name": "Cat1_Thing1" }
]
},
{
"category_name": "Cat2",
"things":
[
{ "name": "Cat2_Thing3" },
{ "name": "Cat2_Thing4" }
]
}
]
So I would have expected to get an array of "categories" each with an array of "items" which are within that category. Instead, it appears to give me a list of things, sorted by the field I grouped on.
Note: category_name is a column in the thing table.

Answer 1

Try something like
my_grouping = Category.includes(:things).
select("*").
group('categories.id, things.id').
order('name')
=> #<ActiveRecord::Relation [#<Category id: 1, name: "Oranges">, #<Category id: 2, name: "Apples">]>
Though, you'll still have to access the Thing objects via my_grouping.things, they'll already be at your hand, and you won't have to wait for the results. This is likely the sort of interaction you're looking for, vs. mapping them into an actual Array.

Answer 2

One option is to do the grouping in Rails (it returns a hash)
Things.order("name").group_by(&:category_name)
#=> {"cat1" => [thing1,thing2,..], "cat2" => [thing3,thing4,..],..}

Answer 3

ActiveRecord::Base#group performs a SQL GROUP BY. I think, but i'm not sure (depends on your db adapter) that as you don't specify any SELECT clause, you get the first record for each category.
To achieve what you want, there are different ways.
For instance, using #includes :
Category.includes(:things).map do |category|
{
category_name: category.name,
things: things.sort_by(&:name).map{|t| {name: t.name} }
}
end.to_json
Note that the standard (albeit often frowned upon) way to serialize models as json is to use (and override if need be) as_json and to_json. so you would have something along the lines of this :
class Category < ActiveRecord::Base
def as_json( options = {} )
defaults = { only: :name, root: false, include: {links: {only: :name}} }
super( defaults.merge(options) )
end
end
Use it like this :
Category.includes(:links).map(&:to_json)
EDIT
As category_name is only a column, you can do :
Thing.order( :category_name, :name ).sort_by( &:category_name ).map do |category, things|
{ category_name: category, things: things.map{|t| {name: t.name} } }
end.to_json
such thing could belong in the model :
def self.sorted_by_category
order( :category_name, :name ).sort_by( &:category_name ).map do |category, things|
{ category_name: category, things: things.map{|t| {name: t.name} } }
end
end
so you can do :
Thing.sorted_by_category.to_json
this way, you can even scope things further :
Thing.where( foo: :bar ).sorted_by_category.to_json