database_abuses.rb

# assumes class Moviegoer with has_many :movies, :through => :reviews

# in controller method:
@fans = Moviegoer.where("zip = ?", code) # table scan if no index!

# in view:
- @fans.each do |fan|
  - fan.movies.each do |movie|
    // BAD: each time thru this loop causes a new database query!
    %p= movie.title

# better: eager loading of the association in controller.
# Rails automatically traverses the through-association between
# Moviegoers and Movies through Reviews
@fans = Moviegoer.where("zip = ?", code).includes(:movies)
# GOOD: preloading movies reviewed by fans avoids N queries in view.

# BAD: preload association but don't use it in view:
- @fans.each do |fan|
  %p= @fan.name
  // BAD: we never used the :movies that were preloaded!