Raise exception on trying to access fields that weren't retrieved from db

Hi. When using mongoengine in our project, we encountered a problem when we tried to access fields that were not received from db (fields in .exclude () or not in .only ()). Now it returns None or the default value, but we think raising an Exception would be safer, so it will tell us to get these fields to avoid unexpected behavior. As an example I added a check_fields_retrieved: True for Document's meta. So for this Document objects and embedded fields we pass .exclude or .only fields, by which we rise an Exception to avoid unexpected behavior. Do you have any developments in this direction?

Start by tracing how Document and embedded-field access behaves after fields are omitted with .exclude() or restricted with .only(). Review the proposed check_fields_retrieved meta option and define the exception behavior for omitted fields, defaults, and embedded fields. Done means configured documents raise an exception instead of returning None or a default value.