title: How to return an empty ActiveRecord relation?
body: <p>If I have a scope with a lambda and it takes an argument, depending on the value of the argument, I might know that there will not be any matches, but I still want to return a relation, not an empty array:</p>
  
  <pre><code>scope :for_users, lambda { |users| users.any? ? where("user_id IN (?)", users.map(&amp;:id).join(',')) : [] }
  </code></pre>
  
  <p>What I really want is a "none" method, the opposite of "all", that returns a relation that can still be chained, but results in the query being short-circuited.</p>