Some tips using active record, check the documentation

active_record_guide.rb

# http://guides.rubyonrails.org/active_record_querying.html

class Article < ActiveRecord::Base
  has_many :comments, -> { order('posted_at DESC') }
end

##### REORDER #####
Article.find(10).comments.reorder(:name)


##### REVERSE_ORDER #####
Article.all.order('name asc')
Article.all.order("name #{params[direction]}")

Article.all.order(:name).reverse_order

##### N + 1 queries #####
# Problem
clients = Client.limit(10)
 
clients.each do |client|
  puts client.address.postcode
end

# Solution
clients = Client.includes(:address).limit(10)
 
clients.each do |client|
  puts client.address.postcode
end

##### MULTI JOINS #####
Category.joins(articles: [{ comments: :guest }, :tags])

##### Some maths #####
Article.count

Article.count(:name)

Article.count(1)

Article.minimum(:likes)

Article.maximum(:likes)

Article.sum(:likes)

Article.average(:likes) # -> Not working

Article.all.average(:likes)