PERF: Preload voters_count and has_voted (#28808)

These fields are often used when serializing topics which may contain
multiple polls. On average, serializing a poll took 2+N queries where N
is the number of options. This change reduces the number of queries to
3, one for each field (Poll#voters_count, PollOption#voters_count and
Poll#has_voted?).

plugins/poll/app/models/poll.rb

@@ -23,6 +23,16 @@ class Poll < ActiveRecord::Base
   validates :max, numericality: { allow_nil: true, only_integer: true, greater_than: 0 }
   validates :step, numericality: { allow_nil: true, only_integer: true, greater_than: 0 }
 
+  attr_writer :voters_count
+  attr_accessor :has_voted
+
+  after_initialize { @has_voted = {} }
+
+  def reload
+    @has_voted = {}
+    super
+  end
+
   def is_closed?
     closed? || (close_at && close_at <= Time.zone.now)
   end
@@ -37,7 +47,17 @@ class Poll < ActiveRecord::Base
   end
 
   def has_voted?(user)
-    user&.id && poll_votes.where(user_id: user.id).exists?
+    if user&.id
+      return @has_voted[user.id] if @has_voted.key?(user.id)
+
+      @has_voted[user.id] = poll_votes.where(user_id: user.id).exists?
+    end
+  end
+
+  def voters_count
+    return @voters_count if defined?(@voters_count)
+
+    @voters_count = poll_votes.count("DISTINCT user_id")
   end
 
   def can_see_voters?(user)
@@ -47,6 +67,36 @@ class Poll < ActiveRecord::Base
   def ranked_choice?
     type == "ranked_choice"
   end
+
+  def self.preload!(polls, user_id: nil)
+    poll_ids = polls.map(&:id)
+
+    voters_count =
+      PollVote
+        .where(poll_id: poll_ids)
+        .group(:poll_id)
+        .pluck(:poll_id, "COUNT(DISTINCT user_id)")
+        .to_h
+
+    option_voters_count =
+      PollVote
+        .where(poll_option_id: PollOption.where(poll_id: poll_ids).select(:id))
+        .group(:poll_option_id)
+        .pluck(:poll_option_id, "COUNT(*)")
+        .to_h
+
+    polls.each do |poll|
+      poll.voters_count = voters_count[poll.id] || 0
+      poll.poll_options.each do |poll_option|
+        poll_option.voters_count = option_voters_count[poll_option.id] || 0
+      end
+    end
+
+    if user_id
+      has_voted = PollVote.where(poll_id: poll_ids, user_id: user_id).pluck(:poll_id).to_set
+      polls.each { |poll| poll.has_voted[user_id] = has_voted.include?(poll.id) }
+    end
+  end
 end
 
 # == Schema Information

plugins/poll/app/models/poll_option.rb

@@ -3,6 +3,14 @@
 class PollOption < ActiveRecord::Base
   belongs_to :poll
   has_many :poll_votes, dependent: :delete_all
+
+  attr_writer :voters_count
+
+  def voters_count
+    return @voters_count if defined?(@voters_count)
+
+    @voters_count = poll_votes.count
+  end
 end
 
 # == Schema Information

plugins/poll/app/serializers/poll_option_serializer.rb

@@ -8,8 +8,7 @@ class PollOptionSerializer < ApplicationSerializer
   end
 
   def votes
-    # `size` instead of `count` to prevent N+1
-    object.poll_votes.size + object.anonymous_votes.to_i
+    object.voters_count + object.anonymous_votes.to_i
   end
 
   def include_votes?

plugins/poll/app/serializers/poll_serializer.rb

@@ -58,7 +58,7 @@ class PollSerializer < ApplicationSerializer
   end
 
   def voters
-    object.poll_votes.count("DISTINCT user_id") + object.anonymous_voters.to_i
+    object.voters_count + object.anonymous_voters.to_i
   end
 
   def close

plugins/poll/plugin.rb

@@ -182,9 +182,9 @@ after_initialize do
           end
 
         if post_with_polls.present?
-          Poll
-            .where(post_id: post_with_polls)
-            .each do |p|
+          all_polls = Poll.includes(:poll_options).where(post_id: post_with_polls)
+          Poll.preload!(all_polls, user_id: @user&.id)
+          all_polls.each do |p|
             polls[p.post_id] ||= []
             polls[p.post_id] << p
           end

plugins/poll/spec/models/poll_spec.rb

@@ -34,7 +34,7 @@ RSpec.describe ::DiscoursePoll::Poll do
 
       expect(poll.can_see_results?(user)).to eq(false)
       poll.poll_votes.create!(poll_option_id: option.id, user_id: user.id)
-      expect(poll.can_see_results?(user)).to eq(true)
+      expect(poll.reload.can_see_results?(user)).to eq(true)
     end
 
     it "author can see results when results setting is on_vote" do

plugins/poll/spec/requests/topics_controller_spec.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+RSpec.describe PostsController do
+  fab!(:admin) { Fabricate(:admin) }
+  fab!(:topic) { Fabricate(:topic, user: admin) }
+
+  fab!(:post1) { Fabricate(:post, topic:, raw: "[poll]\n- A\n- B\n[/poll]") }
+  fab!(:post2) { Fabricate(:post, topic:, raw: "[poll results=on_vote]\n- A\n- B\n[/poll]") }
+  fab!(:post3) { Fabricate(:post, topic:, raw: "[poll results=on_vote]\n- A\n- B\n[/poll]") }
+  fab!(:post4) { Fabricate(:post, topic:, raw: "[poll results=on_vote]\n- A\n- B\n[/poll]") }
+  fab!(:post5) { Fabricate(:post, topic:, raw: "[poll results=staff_only]\n- A\n- B\n[/poll]") }
+  fab!(:post6) { Fabricate(:post, topic:, raw: "[poll results=staff_only]\n- A\n- B\n[/poll]") }
+  fab!(:post7) { Fabricate(:post, topic:, raw: "[poll visibility=]\n- A\n- B\n[/poll]") }
+
+  describe "#show" do
+    context "when not logged in" do
+      it "does not create N+1 queries to load polls" do
+        queries = track_sql_queries { get "/t/#{topic.id}.json" }
+
+        expect(response.status).to eq(200)
+
+        poll_queries = queries.filter { |q| q =~ /FROM "?poll/ }
+        # Expected queries:
+        #
+        # - load all polls
+        # - load all options
+        # - count votes for each poll
+        # - count votes for each option
+        expect(poll_queries.size).to eq(4)
+      end
+    end
+
+    context "when logged in" do
+      before { sign_in(admin) }
+
+      it "does not create N+1 queries to load polls" do
+        queries = track_sql_queries { get "/t/#{topic.id}.json" }
+
+        poll_queries = queries.filter { |q| q =~ /FROM "?poll/ }
+
+        # Expected queries:
+        #
+        # - all queries listed for "when not logged in"
+        # - query to find out if the user has voted in each poll
+        # - queries to get "serialized voters" (NOT TRACKED)
+        expect(poll_queries.size).to eq(5)
+      end
+    end
+  end
+end