Add task to remove non-existent track_id from Event.

lib/tasks/events.rake

@@ -0,0 +1,16 @@
+namespace :events do
+  desc 'Create demo data using our factories'
+  task fix_wrong_track: :environment do
+    events = Event.all.select { |e| e.track_id && Track.find_by(id: e.track_id).nil? }
+    puts "Will remove track_id from #{ActionController::Base.helpers.pluralize(events.length, 'event')}."
+
+    if events.any?
+      puts "Event IDs: #{events.map(&:id)}"
+      events.each do |event|
+        event.track_id = nil
+        event.save!
+      end
+      puts 'All done!'
+    end
+  end
+end