Active Record

includes

• Read

  How to use includes

• Join

  Users.includes(:contanct_info)`

• Apply where condition to associated models. The references is required to access associated models

  Users.includes(:contanct_info)
  .where("contact_infos.city = 'San Diego'")
  .references(:contanct_info)

• Nested Join

  User.includes(contact_info: [:email])

preload v.s eager_load

• Default using preload, which will load leading model and associated model based on the foreign key

  Users.includes(:contanct_info) will load all users and contanct_infos which has foreign key in users

  SELECT users.* FROM users
  SELECT contact_infos.* FROM contact_infos WHERE contact_infos.id IN (1, 2, 3)

• When using includes with where or references, it will use eager_load and load both whole users and contanct_infos instead. eager_load use Left Join

  Users.includes(:contanct_info)
  .where("contact_infos.city = 'San Diego'")
  .references(:contanct_info)

• The data shows that using :includes will improve the performance significantly when a :preload is invoked, but has the opposite effect for :eager_load in most cases

Query Optimization

Why using size instead of count

• count on an ActiveRecord relation will always try to execute a SQL query

• size checks the length of ActiveRecord without making any SQL query

  Only call COUNT when ActiveRecord is not loaded.
  Change the order and replace count with size to make sure ActiveRecord is loaded beforehand

• Use load to load directly (no lazy loading)
  ActiveRecord::QueryCache only happen when a query is called twice

When to use where and when not??**

• Don’t call scopes on associations when you’re rendering collections
• Don’t put query methods, like where, in instance methods of an ActiveRecord::Base class.
• includes + scopes (and other preloading + association) is a way to reduce the cost of where
• includes uses a concept called “Eager loading”. Eager loading works by preloading every comment for every article beforehand in a temporary cache stored in memory.

• When an Active Record associated is called, the association method will first try to pull the requested association data out of the @association_cache instance variable assuming it has already been loaded into memory.

  Three Active Record Mistake

  What’s N+1 query

  Preload scope to avoid make N+1 queries

When to use present, when not??

• present? actually call !blank?
  Emptiness like [] {} “” then present is useful

  [27] pry(main)> !![]
  => true
  [28] pry(main)> [].present?
  => false
  
  Otherwise,
  
  [33] pry(main)> !!0
  => true
  [34] pry(main)> 0.present?
  => true
  [35] pry(main)> !!nil
  => false
  [36] pry(main)> nil.present?
  => false

find_each v.s all.each

• find_each use batch load which is good for memory while dealing with great amounts of data