Pass serializer instead of model to validator

The `UniqueTogetherValidator` may need to access attributes on the
serializer instead of just the model instance. For example, this is
useful for handling field sources.

rest_framework/validators.py

@@ -97,12 +97,12 @@ class UniqueTogetherValidator:
         self.serializer_field = None
         self.message = message or self.message
 
-    def enforce_required_fields(self, attrs, instance):
+    def enforce_required_fields(self, attrs, serializer):
         """
         The `UniqueTogetherValidator` always forces an implied 'required'
         state on the fields it applies to.
         """
-        if instance is not None:
+        if serializer.instance is not None:
             return
 
         missing_items = {
@@ -113,16 +113,16 @@ class UniqueTogetherValidator:
         if missing_items:
             raise ValidationError(missing_items, code='required')
 
-    def filter_queryset(self, attrs, queryset, instance):
+    def filter_queryset(self, attrs, queryset, serializer):
         """
         Filter the queryset to all instances matching the given attributes.
         """
         # If this is an update, then any unprovided field should
         # have it's value set based on the existing instance attribute.
-        if instance is not None:
+        if serializer.instance is not None:
             for field_name in self.fields:
                 if field_name not in attrs:
-                    attrs[field_name] = getattr(instance, field_name)
+                    attrs[field_name] = getattr(serializer.instance, field_name)
 
         # Determine the filter keyword arguments and filter the queryset.
         filter_kwargs = {
@@ -141,13 +141,10 @@ class UniqueTogetherValidator:
         return queryset
 
     def __call__(self, attrs, serializer):
-        # Determine the existing instance, if this is an update operation.
-        instance = getattr(serializer, 'instance', None)
-
-        self.enforce_required_fields(attrs, instance)
+        self.enforce_required_fields(attrs, serializer)
         queryset = self.queryset
-        queryset = self.filter_queryset(attrs, queryset, instance)
-        queryset = self.exclude_current_instance(attrs, queryset, instance)
+        queryset = self.filter_queryset(attrs, queryset, serializer)
+        queryset = self.exclude_current_instance(attrs, queryset, serializer.instance)
 
         # Ignore validation if any field is None
         checked_values = [
@@ -207,13 +204,11 @@ class BaseUniqueForValidator:
         # same as the serializer field names if `source=<>` is set.
         field_name = serializer.fields[self.field].source_attrs[-1]
         date_field_name = serializer.fields[self.date_field].source_attrs[-1]
-        # Determine the existing instance, if this is an update operation.
-        instance = getattr(serializer, 'instance', None)
 
         self.enforce_required_fields(attrs)
         queryset = self.queryset
         queryset = self.filter_queryset(attrs, queryset, field_name, date_field_name)
-        queryset = self.exclude_current_instance(attrs, queryset, instance)
+        queryset = self.exclude_current_instance(attrs, queryset, serializer.instance)
         if qs_exists(queryset):
             message = self.message.format(date_field=self.date_field)
             raise ValidationError({

tests/test_validators.py

@@ -357,11 +357,16 @@ class TestUniquenessTogetherValidation(TestCase):
             def filter(self, **kwargs):
                 self.called_with = kwargs
 
+        class MockSerializer:
+            def __init__(self, instance):
+                self.instance = instance
+
         data = {'race_name': 'bar'}
         queryset = MockQueryset()
+        serializer = MockSerializer(instance=self.instance)
         validator = UniqueTogetherValidator(queryset, fields=('race_name',
                                                               'position'))
-        validator.filter_queryset(attrs=data, queryset=queryset, instance=self.instance)
+        validator.filter_queryset(attrs=data, queryset=queryset, serializer=serializer)
         assert queryset.called_with == {'race_name': 'bar', 'position': 1}