Adam Niedzielski Ruby on Rails’ mistake here

Thoughtbot’s where.not https://robots.thoughtbot.com/activerecords-wherenot

[SO] Difference between where & find http://stackoverflow.com/questions/9574659/rails-where-vs-find

Difference between flatten, flat_map, flatten! http://gistflow.com/posts/578-use-flat_map-instead-of-map-flatten