database_abuses.rb

# assumes class Moviegoer with has_many :favorite_movies, :class_name => 'Movie'

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

# 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
@fan = Moviegoer.find_by_email(email, :include => :favorite_movies)
# now we have preloaded all the associated movies

# in view:
- @fan.favorite_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!