title: MongoEngine _types and _cls fields
body: <p>Why does mongoengine add <strong>_types</strong> and <strong>_cls</strong> fields to every document of a collection.<br>
  Both of them are a (key, value) pair and both of them contain the name of the document's model class. The only difference is <strong>_types</strong> value is a list and I assume it can have multiple model class names if there is involved some inheritance.  </p>
  
  <p>However the question is: why do I need them to exist in every document within a collection when all the documents will have the same values for both fields?  </p>