How to "desc" order group_by? - ruby-on-rails

Question

How to order("created_at desc") the months?
The articles within the month are in descending order, but not the months themselves.
controller
def archives
#posts = Post.order("created_at desc")
#posts_by_months = #posts.group_by { |t| t.created_at.beginning_of_month }
end
view
<% #posts_by_months.sort.each do |month, posts| %>
<%= month.strftime('%b') %>
<% for post in posts %>
<%= post.title %>
<% end %>
<% end %>

Answer 1

Using Enumerable#inject http://ruby-doc.org/core-2.3.1/Enumerable.html#method-i-inject:
#posts_by_months = #posts_by_months.inject({}) do |h,(k,v)|
h[k] = v.sort do |x,y|
y.created_at <=> x.created_at
end
h
end
For example:
irb(main):054:0> hash = #posts_by_months.inject({}) {|h,(k,v)| h[k] = v.sort {|x,y| y.created_at <=> x.created_at}; h}
#=> […]
irb(main):055:0> pp hash.first.map(&:created_at)
[Wed, 08 Jun 2016 22:26:34 UTC +00:00,
Wed, 08 Jun 2016 21:49:49 UTC +00:00,
Wed, 08 Jun 2016 18:30:44 UTC +00:00,
Wed, 08 Jun 2016 18:25:40 UTC +00:00]
UPDATE
Works for Rails view via controller.
# app/controllers/website_controller.rb
class WebsiteController < ApplicationController
def test
#posts = Post.order("created_at desc")
#posts_by_months = #posts.group_by {|t| t.created_at.beginning_of_month}
#posts_by_months = #posts_by_months.inject({}) do |h,(k,v)|
h[k] = v.sort do |x,y|
y.created_at <=> x.created_at
end
h
end
render(:layout => false, :template => 'website/test')
end
end
Using HAML (http://haml.info) template:
# app/views/website/test.html.haml
- #posts_by_months.sort.each do |month, posts|
= month.strftime('%b')
%br
%ul
- for post in posts
%li
= post.title

Answer 2

When sorting by month numbers you have to do some explicit conversions:
In controller:
#posts_by_months = #posts.group_by { |t| t.created_at.beginning_of_month }.
sort_by { |k, _| k.strftime('%-m').to_i }.reverse
#posts_by_months.each { |month, posts| puts month.strftime('%b') } ;
=> Dec
Nov
Oct
Sep
Aug
Jul
Jun
May
Apr
Mar
Feb
Jan
Here k.strftime('%-m') extracts month number without padding as a string and to_i converts that to a number. Without the conversion sort_by will apply lexical sorting which is not what's required.
Result of sort_by is not a hash but two-dimensional array. This does not affect the view code though.

Answer 3

Just in case you are using PostgreSQL:
#posts = Post.select('extract(month from created_at) as month, posts.*').order('month DESC, created_at DESC').group_by(&:month)
#posts.each do |month, posts|
puts "This is the month: #{Date::MONTHNAMES[month]}"
puts "And this is array of posts: #{posts}"
end