eager loading is letting me down

rubyonrails-talk
1

Class User has_many Referrals. Referrals have an instance variable 'point_value.' So on my users/index action I want to list the user info, total referrals, and then total points. However, I am getting the notorious N+1 problem when running this action.

USERS CONTROLLER   def index     @users = User.find(:all, :include => :referrals)   end

INDEX VIEW <% for user in @users %>   <tr>     <td><%= h user.email %></td>     <td>Referrals: <%= user.referrals.size %></td>     <td>Points: <%= user.total_points %>

USER.RB MODEL     has_many :referrals,             :foreign_key => "referer_id"

  def total_points     self.referrals.sum :point_value   end

The index action nails me with a nasty N+2 queries (i have six user records in the test db):

Processing UsersController#index (for 127.0.0.1 at 2009-01-02 15:49:16) [GET]   User Load (4.3ms) SELECT * FROM "users"   Referral Load (1.4ms) SELECT "referrals".* FROM "referrals" WHERE ("referrals".referer_id IN (1,2,3,4,6,7)) Rendering template within layouts/users Rendering users/index   SQL (0.3ms) SELECT sum("referrals".point_value) AS sum_point_value FROM "referrals" WHERE ("referrals".referer_id = 1)   SQL (0.3ms) SELECT sum("referrals".point_value) AS sum_point_value FROM "referrals" WHERE ("referrals".referer_id = 2)   SQL (0.3ms) SELECT sum("referrals".point_value) AS sum_point_value FROM "referrals" WHERE ("referrals".referer_id = 3)   SQL (0.2ms) SELECT sum("referrals".point_value) AS sum_point_value FROM "referrals" WHERE ("referrals".referer_id = 4)   SQL (0.3ms) SELECT sum("referrals".point_value) AS sum_point_value FROM "referrals" WHERE ("referrals".referer_id = 6)   SQL (0.3ms) SELECT sum("referrals".point_value) AS sum_point_value FROM "referrals" WHERE ("referrals".referer_id = 7)

Is this a failing of the rails .sum method to look inside the collection or am I doing something wrong?

2

You probably could have said:

  def total_points     self.referrals.to_a.sum(&:point_value)   end

And then #sum is from Enumerable (as extended by ActiveSupport) rather
than ActiveRecord::Calculations::ClassMethods (note the use of .to_a
in there to get a real array rather than an association proxy)

-Rob