Introduce a ConflictsForTalk model and db view

app/models/conflicts_for_talk.rb

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+class ConflictsForTalk < ApplicationRecord
+end

db/migrate/20161013011420_add_conflicts_for_talk_view.rb

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+CONFLICTS_FOR_TALK_VIEW_SQL = <<EOS
+CREATE VIEW "conflicts_for_talks" AS
+SELECT     DISTINCT("left"."talk_id")              AS "talk_id",
+           COUNT("right"."talk_preference_id") - 1 AS "conflicts"
+FROM       "selected_talks"                        AS "left"
+INNER JOIN "selected_talks"                        AS "right"
+ON         "left"."talk_id" = "right"."talk_id"
+GROUP BY   "left"."id";
+EOS
+
+class AddConflictsForTalkView < ActiveRecord::Migration[5.0]
+  def up
+    execute CONFLICTS_FOR_TALK_VIEW_SQL
+  end
+
+  def down
+    execute 'DROP VIEW "conflicts_for_talks"'
+  end
+end