title: Subquerying in MongoEngine
body: <p>Having the sample code:</p>
  
  <pre><code>import mongoengine as mongo
  
  class User(mongo.Document):
      name = mongo.StringField()
      age = mongo.IntField()    
  
  class Post(mongo.Document):
      title = mongo.StringField()
      author = mongo.ReferenceField(User)
  </code></pre>
  
  <p>I want to get all the posts created by users with ages below 30, one way is to make the request in two steps:</p>
  
  <pre><code>users = User.objects(age__lt=30)
  posts = Post.objects(author__in=users)
  </code></pre>
  
  <p>But even this is not totally bad, it is calling and connecting to the database N times unnecessarily. So I want to make the query in just one step, I tried:</p>
  
  <pre><code>posts = Post.objects(author__age__lt=30)
  </code></pre>
  
  <p>But does not work, it just returns me a empty list, not an error. What am I doing wrong?</p>