class DynamicSettings(DynamicEmbeddedDocument):
    known_field = StringField()

class Person(Document):
    name = StringField()
    settings = EmbeddedDocumentField(DynamicSettings)

p = Person(settings=DynamicSettings(known_field="abc", dynamic_field1="123"), name="John").save()

# The following raises a LookUpError
Person.objects(settings__dynamic_field1="123").first()