Fix #3844, refine validator for fields with <source=> kwargs

When serializers has fields with something like `source=user.email`, the
uniqueness validator should check `email` field instead of `user`, cause
`user` is a model object.

rest_framework/validators.py

@@ -35,7 +35,7 @@ class UniqueValidator(object):
         """
         # Determine the underlying model field name. This may not be the
         # same as the serializer field name if `source=<>` is set.
-        self.field_name = serializer_field.source_attrs[0]
+        self.field_name = serializer_field.source_attrs[-1]
         # Determine the existing instance, if this is an update operation.
         self.instance = getattr(serializer_field.parent, 'instance', None)
 
@@ -174,8 +174,8 @@ class BaseUniqueForValidator(object):
         """
         # Determine the underlying model field names. These may not be the
         # same as the serializer field names if `source=<>` is set.
-        self.field_name = serializer.fields[self.field].source_attrs[0]
-        self.date_field_name = serializer.fields[self.date_field].source_attrs[0]
+        self.field_name = serializer.fields[self.field].source_attrs[-1]
+        self.date_field_name = serializer.fields[self.date_field].source_attrs[-1]
         # Determine the existing instance, if this is an update operation.
         self.instance = getattr(serializer, 'instance', None)
 

tests/test_validators.py

@@ -4,6 +4,7 @@ from django.db import models
 from django.test import TestCase
 
 from rest_framework import serializers
+from rest_framework.validators import UniqueValidator
 
 
 def dedent(blocktext):
@@ -22,6 +23,20 @@ class UniquenessSerializer(serializers.ModelSerializer):
         model = UniquenessModel
 
 
+class RelatedModel(models.Model):
+    user = models.OneToOneField(UniquenessModel, on_delete=models.CASCADE)
+    email = models.CharField(unique=True, max_length=80)
+
+
+class RelatedModelSerializer(serializers.ModelSerializer):
+    username = serializers.CharField(source='user.username',
+        validators=[UniqueValidator(queryset=UniquenessModel.objects.all())])  # NOQA
+
+    class Meta:
+        model = RelatedModel
+        fields = ('username', 'email')
+
+
 class AnotherUniquenessModel(models.Model):
     code = models.IntegerField(unique=True)
 
@@ -73,6 +88,16 @@ class TestUniquenessValidation(TestCase):
         self.assertEqual(
             AnotherUniquenessModel._meta.get_field('code').validators, [])
 
+    def test_related_model_is_unique(self):
+        data = {'username': 'existing', 'email': 'new-email@example.com'}
+        rs = RelatedModelSerializer(data=data)
+        self.assertFalse(rs.is_valid())
+        self.assertEqual(rs.errors,
+                         {'username': ['This field must be unique.']})
+        data = {'username': 'new-username', 'email': 'new-email@example.com'}
+        rs = RelatedModelSerializer(data=data)
+        self.assertTrue(rs.is_valid())
+
 
 # Tests for `UniqueTogetherValidator`
 # -----------------------------------