title: Django can't find existing document using ObjectId with mongoengine
body: <p>I'm trying to update a document from my mLab using mongoengine. I have an existing ObjectId from a document but when I try to get it using the obtained id my search list is empty.</p>
  
  <p>Here's my update code:</p>
  
  <pre><code>def update(request):
  
      if request.method == "POST":
  
          _id = request.POST.get('_id',ObjectId())
          geometry = request.POST.get('geometry', '')
          properties = request.POST.get('properties','')
          try:
              route_test = Route.objects(id=ObjectId(_id))
              print(route_test)
              Route.objects(id=ObjectId(_id)).update(set__geometry=geometry, set__properties=properties)
              return HttpResponse("Success!")
  
          except:
              return HttpResponse("Error!")
  </code></pre>
  
  <p>my <code>route_test</code> would contain nothing : [ ]. Is something wrong with my query? I even tried the following variation: </p>
  
  <p><code>Route.objects(id=_id), 
   Route.objects(_id=ObjectId(_id))</code></p>