title: Combining URL and POST variables in ServiceStack
body: <p>I am trying to convert an existing wcf rest api to ServiceStack, and having issues right out of the gate:</p>
  
  <pre><code>[Route("foo/{userId}","POST")]
  public class MyInputModel : IReturnVoid
  {
      public string userId { get; set; }
      public SomeOtherObject properties { get; set; }
  }
  </code></pre>
  
  <p>The intention here is that I would provide the userId in the url, and an instance of SomeOtherObject in the post body. The error I get is </p>
  
  <pre><code>&lt;Message&gt;Could not deserialize 'application/xml' request using MyInputModel'
  Error: System.Runtime.Serialization.SerializationException: 
  Error in line 1 position 42. Expecting element 'MyInputModel' 
  from namespace 'blahblahblah'.. Encountered 'Element'  with name 
  'SomeOtherObject', namespace 'http://blahblahblah'. 
  </code></pre>
  
  <p>The only things I can think of are to wrap my xml in a <code>MyInputModel</code> to make the serializer happy. This is not really an option for backwards compatibility. </p>
  
  <p>I could also modify <code>SomeOtherObject</code> to be the top level input model, and put a <code>UserId</code> property in there, but this also feels suboptimal since it is an object used throughout the api, and is really not tied to a user id. It is also already published independently, so it would be painful to make changes there. </p>
  
  <p>Is there any way to indicate that the root element of the posted data will be a <code>SomeOtherObject</code> insted of a <code>MyInputModel</code>? In WebApi this would be with the <code>[FromBody]</code> attributes and whatnot. Does servicestack have anything similar?</p>