Small patch to ActiveRecord to make smart queries by rewriting Array#[] for ActiveRelation. It's using SQL limit and offset to get only the relevant objects. Default behavior of ActiveRecord is to get all objects in an array and only after take a subset of this array using Array#[].
Simply add activerelation_limit.rb to config/initializers/ in a Rails App.
ActiveRelaltion#[3] # add limit 1 offset 3 to SQL Query
ActiveRelaltion#[3..5] # add limit 2 offset 3 to SQL Query
Similar patch for Mongoid : https://github.com/hartator/Performance-Hack-for-Mongoid