Fixed. The short version of the story is that rails' .sum helper hits
the DB N+1 times. Writing a custom sum method solves this problem. In
my case, the top method only hits the DB twice whereas the second method
hits the DB N+2 times.
# 2 queries
def total_points
sum = 0
self.referrals.each {|ref| sum += ref.point_value}
sum
end
# N+2 queries
def total_points
self.referrals.sum :point_value
end
So :include DOES work as long as you are careful to manipulate the
collection in ruby and not use another ActiveRecord method to do
summation.
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