Note when reading book The Complete Guild to Rails Performance.

ActiveRecord

• Operations on many records

  # bad
  Rubygem.all.each { | g |
    g.name.upcase!
    g.save
  end
  
  # good
  # default load 1000 records and update it one by one (1000 SQL UPDATE)
  Rubygem.all.find_each do |g|
    g.name.upcase!
    g.save
  end
  
  # good
  # default load 1000 records and update it all in one time (1 SQL UPDATE)
  Rubygem.where("downloads > 100").in_batches do |relation|
    relation.update_all(popular_gem: true)
  end
  
  # Note: Model.update will trigger callback, while Model.update_all skip callbacks
  

• N+1 query
• Replace Query Methods With Enumerable

    # bad
    gem_count = Rubygem.count
    gem_names = Rubygem.pluck(:name)
    "There are #{gem_count} gems, list: #{gem_names}"
  
    # good
    gem_names = Rubygem.pluck(:name)
    "There are #{gem_names.count} gems, list: #{gem_names}"
  

• Select Only What You Need

    # target: get all gem names
    # bad
    Rubygem.all.map(&name)
  
    # good
    Rubygem.all.select(:name).map(&name)
  
    # better
    Rubygem.pluck :name
  

• Take it Easy With Lazy Loading
  • eager_load, use LEFT OUTER JOIN
  • preload, the slowest of all the eager loading method
  • includes, decide for you to use eager_load or preload
• Do Math In The Database

    # bad
    downloads = Rubygem.pluck(:download)
    average = downloads.sum / downloads.count
  
    # good
    Rubygem.average(:download).to_i
  

  Other methods in ActiveRecord::Calculations : average, calculate, count, ids, maximum, minimum, pluck, sum

• Don’t Use Many Queries When One Will Do
  • bulk insert data: use gem activerecord-import
  • update_all, destroy_all will do one SQL statement

Background Job

• Idempotent

    class UserSignupMailJob < ActiveJob::Base
      queue_as :default
  
      around_perform do |job, block|
        user = job.arguments.first
        user.with_lock do
          return if user.signup_email_sent
          if block.call
            user.update_attributes(signup_email_sent: true)
          else
            retry_now # or implement your own backoff procedure
          end
        end
      end
  
      def perform(user)
        UserMailer.signup_email(to: user).deliver
      end
    end