title: Python Eve - where clause using objectid
body: <p>I have the following resource defined in settings.py,</p>
  
  <pre><code>builds = {
      'item_title': 'builds',
      'schema': {
          'sources': {
              'type': 'list',
              'schema': {
                  'type': 'objectid',
                  'data_relation': {
                      'resource': 'sources',
                      'embeddable': True,
                  }
              }
          },
          'checkin_id': {
              'type': 'string',
              'required': True,
              'minlength': 1,
          },
      }
  }
  </code></pre>
  
  <p>When I try to filter based on a member whose value is an objectid, I get empty list.</p>
  
  <pre><code>http://127.0.0.1:5000/builds?where={"sources":"54e328ec537d3d20bbdf2ed5"}
  </code></pre>
  
  <p>54e328ec537d3d20bbdf2ed5 is the id of source</p>
  
  <p>Is there anyway to do this?</p>