bugfix SlugRelatedField with a model property target #943

drf_spectacular/openapi.py

@@ -697,9 +697,16 @@ def _map_serializer_field(self, field, direction, bypass_extensions=False):
                 # be graceful and default to string.
                 model_field = follow_field_source(model, source, default=models.TextField())
 
+            # Special case: SlugRelatedField also allows to point to a callable @property.
+            if callable(model_field):
+                schema = self._map_response_type_hint(model_field)
+            elif isinstance(model_field, models.Field):
+                schema = self._map_model_field(model_field, direction)
+            else:
+                assert False, f'Field "{field.field_name}" must point to either a property or a model field.'
+
             # primary keys are usually non-editable (readOnly=True) and map_model_field correctly
             # signals that attribute. however this does not apply in the context of relations.
-            schema = self._map_model_field(model_field, direction)
             schema.pop('readOnly', None)
             return append_meta(schema, meta)
 

tests/test_regressions.py

@@ -3008,3 +3008,31 @@ class XViewset(viewsets.ModelViewSet):
             'default': []
         }
     }
+
+
+def test_slug_related_field_to_model_property(no_warnings):
+    class M10(models.Model):
+        @property
+        def property_field(self) -> float:
+            return 42
+
+    class M11(models.Model):
+        field = models.ForeignKey(M10, on_delete=models.CASCADE)
+
+    class XSerializer(serializers.ModelSerializer):
+        # How the field is defined in a Serializer
+        field = serializers.SlugRelatedField(slug_field="property_field", read_only=True)
+
+        class Meta:
+            fields = '__all__'
+            model = M11
+
+    class XViewset(viewsets.ModelViewSet):
+        serializer_class = XSerializer
+        queryset = M11.objects.all()
+
+    schema = generate_schema('/x', XViewset)
+    assert schema['components']['schemas']['X']['properties'] == {
+        'id': {'type': 'integer', 'readOnly': True},
+        'field': {'format': 'double', 'readOnly': True, 'type': 'number'}
+    }