title: Differences between `any?` and `exists?` in Ruby on Rails?
body: <p>In Ruby on Rails, there appear to be two methods to check whether a collection has any elements in it.</p>
  
  <p>Namely, they are <a href="http://api.rubyonrails.org/v4.2.10/classes/ActiveRecord/FinderMethods.html#method-i-exists-3F" rel="noreferrer">ActiveRecord::FinderMethods’ exists?</a> and <a href="http://api.rubyonrails.org/v4.2.10/classes/ActiveRecord/Relation.html#method-i-any-3F" rel="noreferrer">ActiveRecord::Relation’s any?</a>. Running these in a generic query (<code>Foo.first.bars.exists?</code> and <code>Foo.first.bars.any?</code>) generated equivalent SQL. Is there any reason to use one over the other?</p>