add test and another case to #901

drf_spectacular/openapi.py

@@ -964,10 +964,10 @@ def _get_serializer_field_meta(self, field, direction):
                     serializers.ModelField,
                     serializers.SerializerMethodField,
                     serializers.PrimaryKeyRelatedField,
+                    serializers.SlugRelatedField,
                 ),
             ):
-                # Skip coercion for lack of a better solution. ModelField.to_representation()
-                # and SerializerMethodField.to_representation() are special in that they require
+                # Skip coercion for lack of a better solution. These are special in that they require
                 # a model instance or object (which we don't have) instead of a plain value.
                 default = field.default
             else:

tests/test_regressions.py

@@ -3036,3 +3036,38 @@ class XViewset(viewsets.ModelViewSet):
         'id': {'type': 'integer', 'readOnly': True},
         'field': {'format': 'double', 'readOnly': True, 'type': 'number'}
     }
+
+
+def test_serializer_foreign_key_default_value_handling(no_warnings):
+    class M10(models.Model):
+        field = models.CharField(unique=True)
+
+    class M11(models.Model):
+        field_related = models.ForeignKey(M10, on_delete=models.CASCADE, default=1)
+
+    class XSerializer(serializers.ModelSerializer):
+        field_related = serializers.PrimaryKeyRelatedField(
+            queryset=M11.objects.all(),
+            default=1,
+        )
+        field_related_slug = serializers.SlugRelatedField(
+            source='field_related',
+            slug_field='field',
+            queryset=M10.objects.all(),
+            default='foo',
+        )
+
+        class Meta:
+            fields = '__all__'
+            model = M11
+
+    class XViewset(viewsets.ModelViewSet):
+        serializer_class = XSerializer
+        queryset = M11.objects.none()
+
+    schema = generate_schema('/x', XViewset)
+    assert schema['components']['schemas']['X']['properties'] == {
+        'id': {'type': 'integer', 'readOnly': True},
+        'field_related': {'type': 'integer', 'default': 1},
+        'field_related_slug': {'type': 'string', 'default': 'foo'},
+    }