fix: Make the instance variable of child serializer point to the correct list object instead of the entire list when validating ListSerializer

rest_framework/serializers.py

@@ -589,6 +589,11 @@ class ListSerializer(BaseSerializer):
         self.min_length = kwargs.pop('min_length', None)
         assert self.child is not None, '`child` is a required argument.'
         assert not inspect.isclass(self.child), '`child` has not been instantiated.'
+
+        if kwargs.get('instance', []) and kwargs.get('data', []):
+            assert len(kwargs.get("data", [])) == len(
+            kwargs.get("instance", [])), 'Data and instance should have same length'
+
         super().__init__(*args, **kwargs)
         self.child.bind(field_name='', parent=self)
 
@@ -663,7 +668,10 @@ class ListSerializer(BaseSerializer):
         ret = []
         errors = []
 
-        for item in data:
+        for idx, item in enumerate(data):
+            if hasattr(self, 'instance') and self.instance and \
+                    len(self.instance) > idx:
+                self.child.instance = self.instance[idx]
             try:
                 validated = self.child.run_validation(item)
             except ValidationError as exc:

tests/test_serializer.py

@@ -762,3 +762,66 @@ class Test8301Regression:
 
         assert (s.data | {}).__class__ == s.data.__class__
         assert ({} | s.data).__class__ == s.data.__class__
+
+
+class MyClass(models.Model):
+    name = models.CharField(max_length=100)
+    value = models.CharField(max_length=100, blank=True)
+
+    app_label = "test"
+
+    @property
+    def is_valid(self):
+        return self.name == 'valid'
+
+
+class MyClassSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = MyClass
+        fields = ('id', 'name', 'value')
+
+    def validate_value(self, value):
+        if value and not self.instance.is_valid:
+            raise serializers.ValidationError(
+                'Status cannot be set for invalid instance')
+        return value
+
+
+import unittest
+
+
+class TestMultipleObjectsValidation(unittest.TestCase):
+    def setUp(self):
+        self.objs = [
+            MyClass(name='valid'),
+            MyClass(name='invalid'),
+            MyClass(name='other'),
+        ]
+
+    def test_multiple_objects_are_validated_separately(self):
+
+        serializer = MyClassSerializer(
+            data=[{'value': 'set', 'id': instance.id} for instance in
+                  self.objs],
+            instance=self.objs,
+            many=True,
+            partial=True,
+        )
+
+        assert not serializer.is_valid()
+        assert serializer.errors == [
+            {},
+            {'value': ['Status cannot be set for invalid instance']},
+            {'value': ['Status cannot be set for invalid instance']}
+        ]
+
+    def test_exception_raised_when_data_and_instance_length_different(self):
+
+        with self.assertRaises(AssertionError):
+            serializer = MyClassSerializer(
+                data=[{'value': 'set', 'id': instance.id} for instance in
+                      self.objs],
+                instance=self.objs[:-1],
+                many=True,
+                partial=True,
+            )