title: How to find entities with empty relationships in Core Data?
body: <p>I have model with entities: <code>PBOUser</code> and <code>PBOLocation</code>.<br>
   There is relationship <code>many to many</code>. Every user may have a lot of locations and every location may belong to many users.</p>
  
  <p>User and location has its own unique field: <code>identifier</code>.</p>
  
  <p>It is very simple to find locations that belongs to a specific user:</p>
  
  <pre><code>let predicate = NSPredicate(format: "ANY users.identifier == %@", user.identifier)
  </code></pre>
  
  <p><em>if any user inside <code>users</code> field has identifier same as user.identifier... this is location I am looking for</em></p>
  
  <p>But the question is:<br>
  <strong>How to find locations that not belong to any user?.</strong> In other words, their <code>users</code> field is empty.</p>