Use a metaclass, generate fields on definition time, _fields_cache -> base_fields

docs/api-guide/serializers.md

@@ -587,7 +587,7 @@ For full details see the [serializer relations][relations] documentation.
 
 ## Customizing field mappings
 
-The ModelSerializer class also exposes a set of class attributes that you can override in order to alter how serializer fields are automatically determined when first instantiating the serializer.
+The ModelSerializer class also exposes a set of class attributes that you can override in order to alter how serializer fields are automatically determined when generating the fields.
 
 Normally if a `ModelSerializer` does not generate the fields you need by default then you should either add them to the class explicitly, or simply use a regular `Serializer` class instead. However in some cases you may want to create a new base class that defines how the serializer fields are created for any given model.
 

rest_framework/serializers.py

@@ -851,7 +851,97 @@ def raise_errors_on_nested_writes(method_name, serializer, validated_data):
     )
 
 
-class ModelSerializer(Serializer):
+class ModelSerializerMetaclass(SerializerMetaclass):
+    """
+    This metaclass sets a dictionary named `base_fields` on the class.
+
+    `base_fields` include the declared fields (see SerializerMetaclass),
+    and fields generated for the model. A deepcopy of `base_fields` is
+    used for each instance of the ModelSerializer.
+    """
+
+    @classmethod
+    def _get_fields(cls, new_class):
+        # XXX
+        if not hasattr(new_class, 'Meta'):
+            return OrderedDict()
+
+        assert hasattr(new_class, 'Meta'), (
+            'Class {serializer_class} missing "Meta" attribute'.format(
+                serializer_class=new_class.__name__
+            )
+        )
+        assert hasattr(new_class.Meta, 'model'), (
+            'Class {serializer_class} missing "Meta.model" attribute'.format(
+                serializer_class=new_class.__name__
+            )
+        )
+        if model_meta.is_abstract_model(new_class.Meta.model):
+            raise ValueError(
+                'Cannot use ModelSerializer with Abstract Models.'
+            )
+
+        if new_class.url_field_name is None:
+            new_class.url_field_name = api_settings.URL_FIELD_NAME
+
+        declared_fields = new_class._declared_fields
+        model = getattr(new_class.Meta, 'model')
+        depth = getattr(new_class.Meta, 'depth', 0)
+
+        if depth is not None:
+            assert depth >= 0, "'depth' may not be negative."
+            assert depth <= 10, "'depth' may not be greater than 10."
+
+        # Retrieve metadata about fields & relationships on the model class.
+        info = model_meta.get_field_info(model)
+        field_names = new_class.get_field_names(declared_fields, info)
+
+        # Determine any extra field arguments and hidden fields that
+        # should be included
+        extra_kwargs = new_class.get_extra_kwargs()
+        extra_kwargs, hidden_fields = new_class.get_uniqueness_extra_kwargs(
+            field_names, declared_fields, extra_kwargs
+        )
+
+        # Determine the fields that should be included on the serializer.
+        fields = OrderedDict()
+
+        for field_name in field_names:
+            # If the field is explicitly declared on the class then use that.
+            if field_name in declared_fields:
+                fields[field_name] = declared_fields[field_name]
+                continue
+
+            extra_field_kwargs = extra_kwargs.get(field_name, {})
+            source = extra_field_kwargs.get('source', '*')
+            if source == '*':
+                source = field_name
+
+            # Determine the serializer field class and keyword arguments.
+            field_class, field_kwargs = new_class.build_field(
+                source, info, model, depth
+            )
+
+            # Include any kwargs defined in `Meta.extra_kwargs`
+            field_kwargs = new_class.include_extra_kwargs(
+                field_kwargs, extra_field_kwargs
+            )
+
+            # Create the serializer field.
+            fields[field_name] = field_class(**field_kwargs)
+
+        # Add in any hidden fields.
+        fields.update(hidden_fields)
+
+        return fields
+
+    def __new__(cls, name, bases, attrs):
+        new_class = super().__new__(cls, name, bases, attrs)
+        new_class.base_fields = cls._get_fields(new_class)
+        return new_class
+
+
+class ModelSerializer(Serializer, metaclass=ModelSerializerMetaclass):
     """
     A `ModelSerializer` is just a regular `Serializer`, except that:
 
@@ -1002,88 +1092,12 @@ class ModelSerializer(Serializer):
 
     # Determine the fields to apply...
 
-    @classmethod
-    def _get_fields(cls):
-        if cls.url_field_name is None:
-            cls.url_field_name = api_settings.URL_FIELD_NAME
-
-        assert hasattr(cls, 'Meta'), (
-            'Class {serializer_class} missing "Meta" attribute'.format(
-                serializer_class=cls.__name__
-            )
-        )
-        assert hasattr(cls.Meta, 'model'), (
-            'Class {serializer_class} missing "Meta.model" attribute'.format(
-                serializer_class=cls.__name__
-            )
-        )
-        if model_meta.is_abstract_model(cls.Meta.model):
-            raise ValueError(
-                'Cannot use ModelSerializer with Abstract Models.'
-            )
-
-        declared_fields = cls._declared_fields
-        model = getattr(cls.Meta, 'model')
-        depth = getattr(cls.Meta, 'depth', 0)
-
-        if depth is not None:
-            assert depth >= 0, "'depth' may not be negative."
-            assert depth <= 10, "'depth' may not be greater than 10."
-
-        # Retrieve metadata about fields & relationships on the model class.
-        info = model_meta.get_field_info(model)
-        field_names = cls.get_field_names(declared_fields, info)
-
-        # Determine any extra field arguments and hidden fields that
-        # should be included
-        extra_kwargs = cls.get_extra_kwargs()
-        extra_kwargs, hidden_fields = cls.get_uniqueness_extra_kwargs(
-            field_names, declared_fields, extra_kwargs
-        )
-
-        # Determine the fields that should be included on the serializer.
-        fields = OrderedDict()
-
-        for field_name in field_names:
-            # If the field is explicitly declared on the class then use that.
-            if field_name in declared_fields:
-                fields[field_name] = declared_fields[field_name]
-                continue
-
-            extra_field_kwargs = extra_kwargs.get(field_name, {})
-            source = extra_field_kwargs.get('source', '*')
-            if source == '*':
-                source = field_name
-
-            # Determine the serializer field class and keyword arguments.
-            field_class, field_kwargs = cls.build_field(
-                source, info, model, depth
-            )
-
-            # Include any kwargs defined in `Meta.extra_kwargs`
-            field_kwargs = cls.include_extra_kwargs(
-                field_kwargs, extra_field_kwargs
-            )
-
-            # Create the serializer field.
-            fields[field_name] = field_class(**field_kwargs)
-
-        # Add in any hidden fields.
-        fields.update(hidden_fields)
-
-        return fields
-
     def get_fields(self):
         """
         Return the dict of field names -> field instances that should be
         used for `self.fields` when instantiating the serializer.
         """
-        cls = self.__class__
+        return copy.deepcopy(self.base_fields)
-        # We don't want the cache to traverse the MRO, since each subclass
-        # has its own fields; hence the cls comparison.
-        if not hasattr(cls, "_fields_cache") or cls._fields_cache[0] != cls:
-            cls._fields_cache = cls, cls._get_fields()
-        return copy.deepcopy(cls._fields_cache[1])
 
     # Methods for determining the set of field names to include...
 

tests/test_model_serializer.py

@@ -144,18 +144,12 @@ class TestModelSerializer(TestCase):
             class Meta:
                 abstract = True
 
-        class TestSerializer(serializers.ModelSerializer):
-            class Meta:
-                model = AbstractModel
-                fields = ('afield',)
-
-        serializer = TestSerializer(data={
-            'afield': 'foo',
-        })
-
         msginitial = 'Cannot use ModelSerializer with Abstract Models.'
         with self.assertRaisesMessage(ValueError, msginitial):
-            serializer.is_valid()
+            class TestSerializer(serializers.ModelSerializer):
+                class Meta:
+                    model = AbstractModel
+                    fields = ('afield',)
 
 
 class TestRegularFieldMappings(TestCase):
@@ -307,33 +301,29 @@ class TestRegularFieldMappings(TestCase):
         Field names that do not map to a model field or relationship should
         raise a configuration error.
         """
-        class TestSerializer(serializers.ModelSerializer):
-            class Meta:
-                model = RegularFieldsModel
-                fields = ('auto_field', 'invalid')
-
         expected = 'Field name `invalid` is not valid for model `RegularFieldsModel`.'
         with self.assertRaisesMessage(ImproperlyConfigured, expected):
-            TestSerializer().fields
+            class TestSerializer(serializers.ModelSerializer):
+                class Meta:
+                    model = RegularFieldsModel
+                    fields = ('auto_field', 'invalid')
 
     def test_missing_field(self):
         """
         Fields that have been declared on the serializer class must be included
         in the `Meta.fields` if it exists.
         """
-        class TestSerializer(serializers.ModelSerializer):
-            missing = serializers.ReadOnlyField()
-
-            class Meta:
-                model = RegularFieldsModel
-                fields = ('auto_field',)
-
         expected = (
             "The field 'missing' was declared on serializer TestSerializer, "
             "but has not been included in the 'fields' option."
         )
         with self.assertRaisesMessage(AssertionError, expected):
-            TestSerializer().fields
+            class TestSerializer(serializers.ModelSerializer):
+                missing = serializers.ReadOnlyField()
+
+                class Meta:
+                    model = RegularFieldsModel
+                    fields = ('auto_field',)
 
     def test_missing_superclass_field(self):
         """
@@ -921,51 +911,43 @@ class MetaClassTestModel(models.Model):
 
 class TestSerializerMetaClass(TestCase):
     def test_meta_class_fields_option(self):
-        class ExampleSerializer(serializers.ModelSerializer):
-            class Meta:
-                model = MetaClassTestModel
-                fields = 'text'
-
         msginitial = "The `fields` option must be a list or tuple"
         with self.assertRaisesMessage(TypeError, msginitial):
-            ExampleSerializer().fields
+            class ExampleSerializer(serializers.ModelSerializer):
+                class Meta:
+                    model = MetaClassTestModel
+                    fields = 'text'
 
     def test_meta_class_exclude_option(self):
-        class ExampleSerializer(serializers.ModelSerializer):
-            class Meta:
-                model = MetaClassTestModel
-                exclude = 'text'
-
         msginitial = "The `exclude` option must be a list or tuple"
         with self.assertRaisesMessage(TypeError, msginitial):
-            ExampleSerializer().fields
+            class ExampleSerializer(serializers.ModelSerializer):
+                class Meta:
+                    model = MetaClassTestModel
+                    exclude = 'text'
 
     def test_meta_class_fields_and_exclude_options(self):
-        class ExampleSerializer(serializers.ModelSerializer):
-            class Meta:
-                model = MetaClassTestModel
-                fields = ('text',)
-                exclude = ('text',)
-
         msginitial = "Cannot set both 'fields' and 'exclude' options on serializer ExampleSerializer."
         with self.assertRaisesMessage(AssertionError, msginitial):
-            ExampleSerializer().fields
+            class ExampleSerializer(serializers.ModelSerializer):
+                class Meta:
+                    model = MetaClassTestModel
+                    fields = ('text',)
+                    exclude = ('text',)
 
     def test_declared_fields_with_exclude_option(self):
-        class ExampleSerializer(serializers.ModelSerializer):
-            text = serializers.CharField()
-
-            class Meta:
-                model = MetaClassTestModel
-                exclude = ('text',)
-
         expected = (
             "Cannot both declare the field 'text' and include it in the "
             "ExampleSerializer 'exclude' option. Remove the field or, if "
             "inherited from a parent serializer, disable with `text = None`."
         )
         with self.assertRaisesMessage(AssertionError, expected):
-            ExampleSerializer().fields
+            class ExampleSerializer(serializers.ModelSerializer):
+                text = serializers.CharField()
+
+                class Meta:
+                    model = MetaClassTestModel
+                    exclude = ('text',)
 
 
 class Issue2704TestCase(TestCase):
@@ -1177,16 +1159,12 @@ class Issue3674Test(TestCase):
 
 class Issue4897TestCase(TestCase):
     def test_should_assert_if_writing_readonly_fields(self):
-        class TestSerializer(serializers.ModelSerializer):
-            class Meta:
-                model = OneFieldModel
-                fields = ('char_field',)
-                readonly_fields = fields
-
-        obj = OneFieldModel.objects.create(char_field='abc')
-
         with pytest.raises(AssertionError) as cm:
-            TestSerializer(obj).fields
+            class TestSerializer(serializers.ModelSerializer):
+                class Meta:
+                    model = OneFieldModel
+                    fields = ('char_field',)
+                    readonly_fields = fields
         cm.match(r'readonly_fields')