title: How do I write a UNION chain with ActiveRelation?
body: <p>I need to be able to chain an arbitrary number of sub-selects with <code>UNION</code> using ActiveRelation.</p>
  
  <p>I'm a little confused by the ARel implementation of this, since it seems to assume <code>UNION</code> is a binary operation.</p>
  
  <p>However:</p>
  
  <pre><code>( select_statement_a ) UNION ( select_statement_b ) UNION ( select_statement_c )
  </code></pre>
  
  <p>is valid SQL. Is this possible without doing nasty string-substitution?</p>