Propagate model field defaults to ModelSerializer fields

docs/api-guide/fields.md

@@ -47,6 +47,8 @@ If set, this gives the default value that will be used for the field if no input
 
 The `default` is not applied during partial update operations. In the partial update case only fields that are provided in the incoming data will have a validated value returned.
 
+If you're using [Model Serializer](https://www.django-rest-framework.org/api-guide/serializers/#modelserializer) and the corresponding `Model` field declares a `default`, it will be used as the serializer field default.
+
 May be set to a function or other callable, in which case the value will be evaluated each time it is used. When called, it will receive no arguments. If the callable has a `requires_context = True` attribute, then the serializer field will be passed as an argument.
 
 For example:

rest_framework/metadata.py

@@ -11,6 +11,7 @@
 from django.utils.encoding import force_str
 
 from rest_framework import exceptions, serializers
+from rest_framework.fields import empty
 from rest_framework.request import clone_request
 from rest_framework.utils.field_mapping import ClassLookupDict
 
@@ -133,6 +134,10 @@ def get_field_info(self, field):
             if value is not None and value != '':
                 field_info[attr] = force_str(value, strings_only=True)
 
+        default = getattr(field, 'default', None)
+        if default is not None and default is not empty and not callable(default):
+            field_info['default'] = default
+
         if getattr(field, 'child', None):
             field_info['child'] = self.get_field_info(field.child)
         elif getattr(field, 'fields', None):

rest_framework/utils/field_mapping.py

@@ -133,6 +133,9 @@ def get_field_kwargs(field_name, model_field):
     if model_field.has_default() or model_field.blank or model_field.null:
         kwargs['required'] = False
 
+    if model_field.has_default():
+        kwargs['default'] = model_field.default
+
     if model_field.blank and (isinstance(model_field, (models.CharField, models.TextField))):
         kwargs['allow_blank'] = True
 

tests/test_metadata.py

@@ -331,6 +331,16 @@ def test_decimal_field_info_type(self):
         assert field_info['max_digits'] == 18
         assert field_info['decimal_places'] == 4
 
+    def test_default_value_field_info(self):
+        options = metadata.SimpleMetadata()
+        field_info = options.get_field_info(serializers.IntegerField(default=42))
+        assert field_info['default'] == 42
+
+    def test_callable_default_field_info(self):
+        options = metadata.SimpleMetadata()
+        field_info = options.get_field_info(serializers.IntegerField(default=list))
+        assert 'default' not in field_info
+
 
 class TestModelSerializerMetadata(TestCase):
     def test_read_only_primary_key_related_field(self):

tests/test_model_serializer.py

@@ -176,7 +176,7 @@ class Meta:
             TestSerializer\(\):
                 auto_field = IntegerField\(read_only=True\)
                 big_integer_field = BigIntegerField\(.*\)
-                boolean_field = BooleanField\(required=False\)
+                boolean_field = BooleanField\(default=False, required=False\)
                 char_field = CharField\(max_length=100\)
                 comma_separated_integer_field = CharField\(max_length=100, validators=\[<django.core.validators.RegexValidator object>\]\)
                 date_field = DateField\(\)
@@ -185,7 +185,7 @@ class Meta:
                 email_field = EmailField\(max_length=100\)
                 float_field = FloatField\(\)
                 integer_field = IntegerField\(.*\)
-                null_boolean_field = BooleanField\(allow_null=True, required=False\)
+                null_boolean_field = BooleanField\(allow_null=True, default=False, required=False\)
                 positive_integer_field = IntegerField\(.*\)
                 positive_small_integer_field = IntegerField\(.*\)
                 slug_field = SlugField\(allow_unicode=False, max_length=100\)
@@ -212,13 +212,41 @@ class Meta:
                 length_limit_field = CharField\(max_length=12, min_length=3\)
                 blank_field = CharField\(allow_blank=True, max_length=10, required=False\)
                 null_field = IntegerField\(allow_null=True,.*required=False\)
-                default_field = IntegerField\(.*required=False\)
+                default_field = IntegerField\(default=0,.*required=False\)
                 descriptive_field = IntegerField\(help_text='Some help text', label='A label'.*\)
                 choices_field = ChoiceField\(choices=(?:\[|\()\('red', 'Red'\), \('blue', 'Blue'\), \('green', 'Green'\)(?:\]|\))\)
                 text_choices_field = ChoiceField\(choices=(?:\[|\()\('red', 'Red'\), \('blue', 'Blue'\), \('green', 'Green'\)(?:\]|\))\)
         """)
         assert re.search(expected, repr(TestSerializer())) is not None
 
+    def test_default_value_used_when_field_omitted(self):
+        """
+        Model field defaults should be propagated to the serializer field,
+        so that omitted fields fall back to the model default on validation.
+        """
+        class TestSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = FieldOptionsModel
+                fields = ('value_limit_field', 'default_field')
+
+        serializer = TestSerializer(data={'value_limit_field': 5})
+        assert serializer.is_valid(), serializer.errors
+        assert serializer.validated_data == {'value_limit_field': 5, 'default_field': 0}
+
+    def test_default_value_skipped_on_partial_update(self):
+        """
+        Propagated model defaults should not be applied on partial updates.
+        """
+        class TestSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = FieldOptionsModel
+                fields = ('value_limit_field', 'default_field')
+
+        instance = FieldOptionsModel(value_limit_field=5, default_field=10)
+        serializer = TestSerializer(instance, data={'value_limit_field': 6}, partial=True)
+        assert serializer.is_valid(), serializer.errors
+        assert serializer.validated_data == {'value_limit_field': 6}
+
     def test_nullable_boolean_field_choices(self):
         class NullableBooleanChoicesModel(models.Model):
             CHECKLIST_OPTIONS = (