Arel is a SQL abstract syntax tree manager for ruby. It helps to simplify creating complex SQL queries and interfaces with various RDBMS. Arel modifies it's AST nodes based on the methods you call on it and ActiveRecord makes use of this to build composable chainable queries and then executes them. A detailed post on Arel: https://jpospisil.com/2014/06/16/the-definitive-guide-to-arel-the-sql-manager-for-ruby.html