title: MongoEngine raw query not solved
body: <p>Mongo engine cannot execute a raw query</p>
  
  <pre><code>AppDocument.objects(__raw__={
  {"_id": ObjectId("1"),"car._id": ObjectId("2")}, 
              {"$pull": 
                  {"car.$.toys": {"_id": ObjectId("3")}}
              }
  }
  </code></pre>
  
  <p>The error is : </p>
  
  <blockquote>
    <p>TypeError: unhashable type: 'dict'</p>
  </blockquote>