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Fix N+1 Queries

25 Jul 2010

Wen-Tien Chang (ihower@gmail.com)

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.

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