Optimise conflicts calculation

app/models/conflicts.rb

@@ -1,12 +1,9 @@
-class Conflicts
-  include ActiveModel::Model
+class Conflicts < ApplicationRecord
+  def self.most
+    order(conflicts: :desc).try(:first)
+  end
 
-  attr_accessor :left, :right
-
-  def conflicts
-    TalkPreference.joins('INNER JOIN "selected_talks" AS "left"
-                          ON "left"."talk_preference_id" = "talk_preferences"."unique_id"
-                          INNER JOIN "selected_talks" AS "right"
-                          ON "right"."talk_preference_id" = "talk_preferences"."unique_id"').where(left: {talk_id: @left}, right: {talk_id: @right}).count
+  def self.least
+    order(conflicts: :asc).try(:first)
   end
 end

app/models/summary.rb

@@ -10,14 +10,28 @@ class Summary
   end
 
   def most_conflicts
-    ConflictsForTalk.where(talk_id: talk_ids).most.conflicts
+    Conflicts.where(left: talk_ids, right: talk_ids).most.conflicts
   end
 
   def least_conflicts
-    ConflictsForTalk.where(talk_id: talk_ids).least.conflicts
+    if least_conflicts_for_single_talk == 0
+      0
+    else
+      least_conflicts_between_two_talks
+    end
   end
 
   def ranking
     @ranking ||= Ranking.new(talk_ids: talk_ids).ranking
   end
+
+  private
+
+  def least_conflicts_for_single_talk
+    ConflictsForTalk.where(talk_id: talk_ids).least.conflicts
+  end
+
+  def least_conflicts_between_two_talks
+    Conflicts.where(left: talk_ids, right: talk_ids).least.conflicts
+  end
 end

db/migrate/20161013023851_add_conflicts_view.rb

@@ -0,0 +1,19 @@
+CONFLICTS_VIEW_SQL = <<EOS
+CREATE VIEW "conflicts" AS
+SELECT      "left"."talk_id" AS left, "right"."talk_id" AS right, COUNT(*) AS "conflicts"
+FROM        "selected_talks"                              AS "left"
+INNER JOIN  "selected_talks"                              AS "right"
+ON          "left"."talk_preference_id" = "right"."talk_preference_id"
+WHERE       "left"."talk_id" != "right"."talk_id"
+GROUP BY    "left"."talk_id", "right"."talk_id";
+EOS
+
+class AddConflictsView < ActiveRecord::Migration[5.0]
+  def up
+    execute CONFLICTS_VIEW_SQL
+  end
+
+  def down
+    execute 'DROP VIEW "conflicts"'
+  end
+end