title: mongoengine - Ignore extra fields for schema validation
body: <p>I am trying to query my database. Some records currently have extra fields that are not included in my model schema (by error, but I want to handle these cases). When I try to query the DB and transform the records into the schema, I get the following error:</p>
  
  <pre><code>FieldDoesNotExist
  The field 'X' does not exist on the document 'Y'
  </code></pre>
  
  <p>Because of the extra fields in the database that differ from the schema.</p>
  
  <p>Is there a way to ignore this schema validation for extra fields in mongoengine?</p>