database_abuses.rb

# assumes class Moviegoer with has_many :movies

# in controller method:
@fan = Moviegoer.find_by_email(email) # causes table scan if no index

# in view:
- @fan.favorite_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
@fan = Moviegoer.includes(:movies).find_by_email(email)
# now we have preloaded all the movies this moviegoer reviewed.

# in view:
- @fan.movies.each do |movie|
  // GOOD: this code no longer causes additional queries
  %p= movie.title

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