Bring UniqueValidator implementation in line with other uniquness validators.

Author: tomchristie

docs/api-guide/validators.md

@@ -16,9 +16,9 @@ Validation in Django REST framework serializers is handled a little differently
 
 With `ModelForm` the validation is performed partially on the form, and partially on the model instance. With REST framework the validation is performed entirely on the serializer class. This is advantageous for the following reasons:
 
-* It introduces a proper separation of concerns, making your code behaviour more obvious.
-* It is easy to switch between using shortcut `ModelSerializer` classes and using  explicit `Serializer` classes. Any validation behaviour being used for `ModelSerializer` is simple to replicate.
-* Printing the `repr` of a serializer instance will show you exactly what validation rules it applies. There's no extra hidden validation behaviour being called on the model instance.
+* It introduces a proper separation of concerns, making your code behavior more obvious.
+* It is easy to switch between using shortcut `ModelSerializer` classes and using  explicit `Serializer` classes. Any validation behavior being used for `ModelSerializer` is simple to replicate.
+* Printing the `repr` of a serializer instance will show you exactly what validation rules it applies. There's no extra hidden validation behavior being called on the model instance.
 
 When you're using `ModelSerializer` all of this is handled automatically for you. If you want to drop down to using a `Serializer` classes instead, then you need to define the validation rules explicitly.
 

rest_framework/validators.py

@@ -35,12 +35,26 @@ def set_context(self, serializer_field):
         # Determine the existing instance, if this is an update operation.
         self.instance = getattr(serializer_field.parent, 'instance', None)
 
-    def __call__(self, value):
-        # Ensure uniqueness.
+    def filter_queryset(self, value, queryset):
+        """
+        Filter the queryset to all instances matching the given attribute.
+        """
         filter_kwargs = {self.field_name: value}
-        queryset = self.queryset.filter(**filter_kwargs)
+        return queryset.filter(**filter_kwargs)
+
+    def exclude_current_instance(self, queryset):
+        """
+        If an instance is being updated, then do not include
+        that instance itself as a uniqueness conflict.
+        """
         if self.instance is not None:
-            queryset = queryset.exclude(pk=self.instance.pk)
+            return queryset.exclude(pk=self.instance.pk)
+        return queryset
+
+    def __call__(self, value):
+        queryset = self.queryset
+        queryset = self.filter_queryset(value, queryset)
+        queryset = self.exclude_current_instance(queryset)
         if queryset.exists():
             raise ValidationError(self.message)