Rails 3.2.2 Query on sub sub attribute advise - ruby-on-rails-3

Question

Currently have a very inefficient view partial that lists groups of users by their predicted score.
group controller
def show
#sfs_ordered = ScoreFootballSimple.order("home_score DESC, away_score ASC")
#live_games = Game.find(:all, :conditions => ['kickoff < ? AND completed != true AND game_state is NOT NULL', Time.now])
group#show (relevant section)
<% #live_games.each do |game| %>
<% #sfs_ordered.each do |sfs| %>
<% got_this_score = Array.new %>
<% game_points = nil %>
<% #group.members.each do |member| %>
<% if pred = member.prediction_set.predictions.where('game_id = ?',game.id).first %>
<% game_points = pred.points if !pred.points.nil? && pred.score_type == sfs.score_type %>
<% got_this_score << member.user.user_detail.display_name if pred.score_type == sfs.score_type %>
<% end %>
<% end %>
<% if got_this_score.count > 0 %>
<tr><td><%= sfs.home_score %>-<%=sfs.away_score%></td>
<td><% if !game_points.nil? %>
<div class="preds-show-points-div"><%= game_points %>pts</div>
<% else %>
-
<% end%></td>
<td><%= got_this_score.to_sentence %></td></tr>
<% end%>
<% end %>
<% end %>
Obviously this is loops within loops that means the that for every #sfs_ordered (round 50 records) it is iterating over every group member (for the largest group about 5000) and this means the page is taking seconds to load.
Don't flame me, this was the POC to show how it could look but it has exposed my lack of ability with ActiveRecord. Now I could go around creating a hash of users, predictions sets etc but I wondered if anyone could point me to a better way of selecting information with more precision using Rails queries.
The entity relationship is something like this
Group has many Member
Member belongs to User and PredictionSet
PredictionSet has many Prediction
Prediction belongs to Game and ScoreType
ScoreTypeSimple has one ScoreType
The predicted score is in ScoreTypeSimple - this is how I want the list organised
e.g 1:1 - Joe, Fred and Jane
1:0 - Sue, Dave and Helen
Then I want to grab the Group.member.user.name for the Group.member.prediction_set.prediction - where the prediction.game.id == game.id AND score_type == sfs.score_type
I know I can improve this by pure SQL joins and INs and build hashes but I wondered if any one could give me any pointers if there was a Rails/Ruby efficient way of doing this. I know the answer is probably out there in lambda but my ActiveRecord knowledge is stretch to the limit here!
Any help gratefully received.
Peter

Answer 1

You might be able to benefit from eager loading, since you're displaying all of some associated objects. This is done using the .includes method on a relation. For instance, your view works with all the members of a group, so when you fetch the group from the database (I don't see the line that does this in your code, but it probably looks something like
#group = Group.where('some condition').first
If you instead use this:
#group = Group.includes(:members => [:user]).where('some condition').first
Then loading the group, all it's members, and all their user objects is 3 database queries, rather than (in your extreme case of 5000 members) 10,001.
I'd say this is, at best, a small part of your solution, but it may help.
Edit: Here is the RailsCast on eager loading, and there's some docs about half way down this page.