title: Convert relationship to query object
body: <p>Let's say I have many to many relationship between tables User and Car.</p>
  
  <p>It works fine when I use </p>
  
  <pre><code>User.query.filter_by(name='somename').all().cars
  
  Car.query.filter_by(vin='xxxxxx').all().users
  </code></pre>
  
  <p>I have created function that converts BaseQuery to xml object so I need to extract BaseQuery from <code>Car.query.filter_by(vin='xxxxxx').all().users</code>.</p>
  
  <p>Is there any way to do that?</p>