Add null related field option

rest_framework/fields.py

@@ -269,6 +269,7 @@ class RelatedField(WritableField):
 
     def __init__(self, *args, **kwargs):
         self.queryset = kwargs.pop('queryset', None)
+        self.null = kwargs.pop('null', False)
         super(RelatedField, self).__init__(*args, **kwargs)
         self.read_only = kwargs.pop('read_only', self.default_read_only)
 
@@ -351,9 +352,9 @@ class RelatedField(WritableField):
 
         value = data.get(field_name)
 
-        if value in (None, '') and not self.blank:
+        if value in (None, '') and not self.null:
             raise ValidationError('Value may not be null')
-        elif value in (None, '') and self.blank:
+        elif value in (None, '') and self.null:
             into[(self.source or field_name)] = None
         else:
             into[(self.source or field_name)] = self.from_native(value)

rest_framework/serializers.py

@@ -432,7 +432,7 @@ class ModelSerializer(Serializer):
         # TODO: filter queryset using:
         # .using(db).complex_filter(self.rel.limit_choices_to)
         kwargs = {
-            'blank': model_field.blank,
+            'null': model_field.null,
             'queryset': model_field.rel.to._default_manager
         }
 
@@ -577,7 +577,7 @@ class HyperlinkedModelSerializer(ModelSerializer):
         # .using(db).complex_filter(self.rel.limit_choices_to)
         rel = model_field.rel.to
         kwargs = {
-            'blank': model_field.blank,
+            'null': model_field.null,
             'queryset': rel._default_manager,
             'view_name': self._get_default_view_name(rel)
         }