Improve database queries links

https://blog.heroku.com/rails-database-optimization

https://www.speedshop.co/2019/01/10/three-activerecord-mistakes.html https://www.justinweiss.com/articles/how-to-preload-rails-scopes/

https://blog.bigbinary.com/2016/05/30/rails-5-adds-or-support-in-active-record.html https://solidfoundationwebdev.com/blog/posts/how-to-find-records-based-on-has_many-relationship-being-empty-or-not-in-rails http://tomdallimore.com/blog/includes-vs-joins-in-rails-when-and-where/

https://stackoverflow.com/questions/5319400/want-to-find-records-with-no-associated-records-in-rails-3#5570221

https://medium.com/@codenode/10-tips-for-eager-loading-to-avoid-n-1-queries-in-rails-2bad54456a3f