title: ReferenceField with reference to existing document
body: <p>As far as I understood, <a href="http://mongoengine-odm.readthedocs.org/en/latest/apireference.html#mongoengine.fields.ReferenceField" rel="nofollow">mongoengine's ReferenceField</a> allows to pass a reference to another document stored in the DB.</p>
  
  <p>So far I was able to create several documents in different collections (reduced to two in this example), which I need to reference to each other now.</p>
  
  <p>As you will see below I want to pass reference of a given material when creating a new document in the regenerators collection. In the example I provided, this means to pass a reference to the document <code>aisi304</code>.</p>
  
  <p>My regenerator documents are defined as follows:</p>
  
  <pre><code>class Regenerators(Document):
      material = ReferenceField(Materials, required=True)
  </code></pre>
  
  <p>The materials document needs to be referenced to a regenerator document and is defined like this:</p>
  
  <pre><code>class Materials(Document):
      title = StringField(unique=True, required=True)
  </code></pre>
  
  <p>However, the desired materials document is stored in the DB already as in this case:</p>
  
  <pre><code>{
      "_id": ObjectId("565b84dc55c40f63392ffdee"),
      "title": "aisi304"
  }
  </code></pre>
  
  <p>So I tried to create a new regenerator document as follows passing the title of the material document to be referenced as a keyword argument:</p>
  
  <pre><code># the passed keywords are read from json normally, but I put it in directly for the sake of readability
  r = Regenerators(material="aisi304")
  r.save()
  </code></pre>
  
  <p>However, the reference does not seem to be passed correctly since the regernator document stored in the DB looks like this (using mongod for debugging):</p>
  
  <pre><code>{
      "_id": ObjectId("565b89d355c40f6355fa5f45"),
      "material": "aisi304"
  }
  </code></pre>
  
  <p>I thought I would achieve something like this:</p>
  
  <pre><code>{
      "_id": ObjectId("565b89d355c40f6355fa5f45"),
      "material": {
          "_id": ObjectId("565b84dc55c40f63392ffdee"),
          "title": "aisi304"
      }
  }
  </code></pre>
  
  <p>In the <a href="http://docs.mongoengine.org/tutorial.html#adding-data-to-our-tumblelog" rel="nofollow">tutorial provided in the docs</a> they pass a reference to a newly created document. However, this is not suitable for me since I need to reference to existing documents.</p>
  
  <p>What am I doing wrong?</p>