Cannot add new field through lazy migration

[ghost]

It is not possible to add a new field to documents through the lazy migration feature.

If the field is not required, no error is thrown if the field is not in the document. Therefore, the migration is not applied.

If the field is required, a RequireFieldError is raised indicating that the new field is missing on the document. However, this error is not handled in the auto-migration logic

/mongokit/document.py

    235         if auto_migrate:
    236             error = None
    237             try:
    238                 super(Document, self).validate()
    239             except StructureError, e:
    240                 error = e
    241             except KeyError, e:
    242                 error = e
    243             except SchemaTypeError, e:
    244                 error = e
    245             if error:
    246                 if not self.migration_handler:
    247                     raise StructureError(str(error))
    248                 else:
    249                     # if we are here that's becose super.validate failed
    250                     # but it has processed custom type to bson.
    251                     self._migrate(process_to_bson=False)