.validate() can raise field errors or non-field errors

docs/topics/3.0-announcement.md

@@ -163,7 +163,7 @@ The `validate_<field_name>` method hooks that can be attached to serializer clas
             raise serializers.ValidationError('This field should be a multiple of ten.')
         return attrs
 
-This is now simplified slightly, and the method hooks simply take the value to be validated, and return it's validated value.
+This is now simplified slightly, and the method hooks simply take the value to be validated, and return the validated value.
 
     def validate_score(self, value):
         if value % 10 != 0:
@@ -172,6 +172,22 @@ This is now simplified slightly, and the method hooks simply take the value to b
 
 Any ad-hoc validation that applies to more than one field should go in the `.validate(self, attrs)` method as usual.
 
+Because `.validate_<field_name>` would previously accept the complete dictionary of attributes, it could be used to validate a field depending on the input in another field. Now if you need to do this you should use `.validate()` instead.
+
+You can either return `non_field_errors` from the validate method by raising a simple `ValidationError`
+
+    def validate(self, attrs):
+        # serializer.errors == {'non_field_errors': ['A non field error']}
+        raise serailizers.ValidationError('A non field error')
+
+Alternatively if you want the errors to be against a specific field, use a dictionary of when instantiating the `ValidationError`, like so:
+
+    def validate(self, attrs):
+        # serializer.errors == {'my_field': ['A field error']}
+        raise serailizers.ValidationError({'my_field': 'A field error'})
+
+This ensures you can still write validation that compares all the input fields, but that marks the error against a particular field.
+
 #### Limitations of ModelSerializer validation.
 
 This change also means that we no longer use the `.full_clean()` method on model instances, but instead perform all validation explicitly on the serializer. This gives a cleaner separation, and ensures that there's no automatic validation behavior on `ModelSerializer` classes that can't also be easily replicated on regular `Serializer` classes.
@@ -189,7 +205,32 @@ REST framework 2.x attempted to automatically support writable nested serializat
 * It's unclear what behavior the user should expect when related models are passed `None` data.
 * It's unclear how the user should expect to-many relationships to handle updates, creations and deletions of multiple records.
 
-Using the `depth` option on `ModelSerializer` will now create **read-only nested serializers** by default. To use writable nested serialization you'll want to declare a nested field on the serializer class, and write the `create()` and/or `update()` methods explicitly.
+Using the `depth` option on `ModelSerializer` will now create **read-only nested serializers** by default.
+
+If you try to use a writable nested serializer without writing a custom `create()` and/or `update()` method you'll see an assertion error when you attempt to save the serializer. For example:
+
+    >>> class ProfileSerializer(serializers.ModelSerializer):
+    >>>     class Meta:
+    >>>         model = Profile
+    >>>         fields = ('address', 'phone')
+    >>>
+    >>> class UserSerializer(serializers.ModelSerializer):
+    >>>     profile = ProfileSerializer()
+    >>>     class Meta:
+    >>>         model = User
+    >>>         fields = ('username', 'email', 'profile')
+    >>>
+    >>> data = {
+    >>>     'username': 'lizzy',
+    >>>     'email': 'lizzy@example.com',
+    >>>     'profile': {'address': '123 Acacia Avenue', 'phone': '01273 100200'}
+    >>> }
+    >>>
+    >>> serializer = UserSerializer(data=data)
+    >>> serializer.save()
+    AssertionError: The `.create()` method does not suport nested writable fields by default. Write an explicit `.create()` method for serializer `UserSerializer`, or set `read_only=True` on nested serializer fields.
+
+To use writable nested serialization you'll want to declare a nested field on the serializer class, and write the `create()` and/or `update()` methods explicitly.
 
     class UserSerializer(serializers.ModelSerializer):
         profile = ProfileSerializer()

rest_framework/serializers.py

@@ -99,10 +99,10 @@ class BaseSerializer(Field):
 
     def is_valid(self, raise_exception=False):
         assert not hasattr(self, 'restore_object'), (
-            'Serializer %s has old-style version 2 `.restore_object()` '
+            'Serializer `%s.%s` has old-style version 2 `.restore_object()` '
             'that is no longer compatible with REST framework 3. '
             'Use the new-style `.create()` and `.update()` methods instead.' %
-            self.__class__.__name__
+            (self.__class__.__module__, self.__class__.__name__)
         )
 
         if not hasattr(self, '_validated_data'):
@@ -341,9 +341,22 @@ class Serializer(BaseSerializer):
             value = self.validate(value)
             assert value is not None, '.validate() should return the validated data'
         except ValidationError as exc:
+            if isinstance(exc.detail, dict):
+                # .validate() errors may be a dict, in which case, use
+                # standard {key: list of values} style.
+                raise ValidationError(dict([
+                    (key, value if isinstance(value, list) else [value])
+                    for key, value in exc.detail.items()
+                ]))
+            elif isinstance(exc.detail, list):
                 raise ValidationError({
                     api_settings.NON_FIELD_ERRORS_KEY: exc.detail
                 })
+            else:
+                raise ValidationError({
+                    api_settings.NON_FIELD_ERRORS_KEY: [exc.detail]
+                })
+
         return value
 
     def to_internal_value(self, data):
@@ -507,14 +520,17 @@ class ModelSerializer(Serializer):
                 self._kwargs['validators'] = validators
 
     def create(self, validated_attrs):
+        # Check that the user isn't trying to handle a writable nested field.
+        # If we don't do this explicitly they'd likely get a confusing
+        # error at the point of calling `Model.objects.create()`.
         assert not any(
             isinstance(field, BaseSerializer) and not field.read_only
             for field in self.fields.values()
         ), (
             'The `.create()` method does not suport nested writable fields '
             'by default. Write an explicit `.create()` method for serializer '
-            '%s, or set `read_only=True` on nested serializer fields.' %
-            self.__class__.__name__
+            '`%s.%s`, or set `read_only=True` on nested serializer fields.' %
+            (self.__class__.__module__, self.__class__.__name__)
         )
 
         ModelClass = self.Meta.model
@@ -544,8 +560,8 @@ class ModelSerializer(Serializer):
         ), (
             'The `.update()` method does not suport nested writable fields '
             'by default. Write an explicit `.update()` method for serializer '
-            '%s, or set `read_only=True` on nested serializer fields.' %
-            self.__class__.__name__
+            '`%s.%s`, or set `read_only=True` on nested serializer fields.' %
+            (self.__class__.__module__, self.__class__.__name__)
         )
 
         for attr, value in validated_attrs.items():

tests/test_serializer.py

@@ -43,6 +43,32 @@ class TestSerializer:
             serializer.data
 
 
+class TestValidateMethod:
+    def test_non_field_error_validate_method(self):
+        class ExampleSerializer(serializers.Serializer):
+            char = serializers.CharField()
+            integer = serializers.IntegerField()
+
+            def validate(self, attrs):
+                raise serializers.ValidationError('Non field error')
+
+        serializer = ExampleSerializer(data={'char': 'abc', 'integer': 123})
+        assert not serializer.is_valid()
+        assert serializer.errors == {'non_field_errors': ['Non field error']}
+
+    def test_field_error_validate_method(self):
+        class ExampleSerializer(serializers.Serializer):
+            char = serializers.CharField()
+            integer = serializers.IntegerField()
+
+            def validate(self, attrs):
+                raise serializers.ValidationError({'char': 'Field error'})
+
+        serializer = ExampleSerializer(data={'char': 'abc', 'integer': 123})
+        assert not serializer.is_valid()
+        assert serializer.errors == {'char': ['Field error']}
+
+
 class TestBaseSerializer:
     def setup(self):
         class ExampleSerializer(serializers.BaseSerializer):