Fix N+1 Queries

http://rails-bestpractices.com/posts/29-fix-n-1-queries


N+1 Queries is a serious database performance problem. Be careful of that situation! If you're not sure, I recommend you install http://github.com/flyerhzm/bullet
 plugin, which helps you reduce the number of queries with alerts (and growl).
Sample Model code

# model
class User < ActieRecord::Base
    has_one :car
end
-
class Car < ActiveRecord::Base
    belongs_to :user
end

Bad Smell

# your controller
def index
  @users = User.paginate( :page => params[:page], :per_page => 20 )
end

# view
<% @users.each do |user| %>
   <%= user.car.name %>
<% end %>

When @users.each in view, it cause N+1 queries. (N is 20 in this case)
Refactor

# your controller
def index
  @users = User.paginate( :include => :car, :page => params[:page], :per_page => 20 )
end

Add :include into query, so the number of queries reduces to only 2.

总结： 某种时候include  是必须的