Explicit validation of custom field name unique constraint

src/documents/serialisers.py

@@ -441,6 +441,17 @@ class CustomFieldSerializer(serializers.ModelSerializer):
             "data_type",
         ]
 
+    def validate(self, attrs):
+        # see https://github.com/encode/django-rest-framework/issues/7173
+        name = attrs["name"] if "name" in attrs else self.instance.name
+        if ("name" in attrs) and self.Meta.model.objects.filter(
+            name=name,
+        ).exists():
+            raise serializers.ValidationError(
+                {"error": "Object violates name unique constraint"},
+            )
+        return super().validate(attrs)
+
 
 class ReadWriteSerializerMethodField(serializers.SerializerMethodField):
     """

src/documents/tests/test_api_custom_fields.py

@@ -53,6 +53,29 @@ class TestCustomField(DirectoriesMixin, APITestCase):
             self.assertEqual(data["name"], name)
             self.assertEqual(data["data_type"], field_type)
 
+    def test_create_custom_field_nonunique_name(self):
+        """
+        GIVEN:
+            - Custom field exists
+        WHEN:
+            - API request to create custom field with the same name
+        THEN:
+            - HTTP 400 is returned
+        """
+        CustomField.objects.create(
+            name="Test Custom Field",
+            data_type=CustomField.FieldDataType.STRING,
+        )
+
+        resp = self.client.post(
+            self.ENDPOINT,
+            data={
+                "data_type": "string",
+                "name": "Test Custom Field",
+            },
+        )
+        self.assertEqual(resp.status_code, status.HTTP_400_BAD_REQUEST)
+
     def test_create_custom_field_instance(self):
         """
         GIVEN: