Fix DBRefField not handing null values

tbone/data/fields/base.py

@@ -254,7 +254,7 @@ def __call__(self, value):
 
     def __repr__(self):
         if self._bound:
-            return '<%s instance in model %s>' % (self.__class__.__qualname__, self.container_model_class.__name__)
+            return '<%s instance in model %s>' % (self.__class__.__qualname__, self.container_model.__name__)
         return '<%s instance>' % self.__class__.__qualname__
 
     @property

tbone/data/fields/mongo.py

@@ -53,6 +53,8 @@ def __init__(self, model_class, **kwargs):
             raise TypeError("DBRefField: Expected a model of the type '{}'.".format(model_class.__name__))
 
     def _import(self, value):
+        if value is None:
+            return None
         if isinstance(value, self.model_class):
             if not hasattr(value, '_id'):
                 raise ValueError(self._errors['missing_id'])
@@ -68,6 +70,8 @@ def _import(self, value):
         raise ValueError(self._errors['to_python'])
 
     def _export(self, value):
+        if value is None:
+            return None
         if isinstance(value, self.model_class):
             if not hasattr(value, '_id'):
                 raise ValueError(self._errors['missing_id'])