title: Combine two ActiveRecord::Relation objects
body: <p>Suppose I have the following two objects:</p>
  
  <pre><code>first_name_relation = User.where(:first_name =&gt; 'Tobias') # ActiveRecord::Relation
  last_name_relation  = User.where(:last_name  =&gt; 'Fünke') # ActiveRecord::Relation
  </code></pre>
  
  <p>is it possible to combine the two relations to produce one <code>ActiveRecord::Relation</code> object containing both conditions?</p>
  
  <p><em>Note: I'm aware that I can chain the wheres to get this behavior, what I'm really interested in is the case where I have two separate <code>ActiveRecord::Relation</code> objects.</em></p>