🦺 [#72] Fine tune validations of selects fields

* Ensure multiple false|true is handled appropriately
* Model current behaviour of frontend data in tests
* Ensure that empty options are treated correctly
  depending on required/optional config

src/openforms/formio/components/vanilla.py

@@ -7,7 +7,7 @@
 
 import logging
 from datetime import time
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Any
 
 from django.core.validators import MaxValueValidator, MinValueValidator, RegexValidator
 from django.utils.translation import gettext_lazy as _
@@ -440,16 +440,31 @@ def localize(self, component: SelectComponent, language_code: str, enabled: bool
 
     def build_serializer_field(
         self, component: SelectComponent
-    ) -> serializers.MultipleChoiceField:
+    ) -> serializers.ChoiceField:
         validate = component.get("validate", {})
         required = validate.get("required", False)
+        assert "values" in component["data"]
         choices = [
             (value["value"], value["label"]) for value in component["data"]["values"]
         ]
-        return serializers.MultipleChoiceField(
+
+        # map multiple false/true to the respective serializer field configuration
+        field_kwargs: dict[str, Any]
+        match component:
+            case {"multiple": True}:
+                field_cls = serializers.MultipleChoiceField
+                field_kwargs = {"allow_empty": not required}
+            case _:
+                field_cls = serializers.ChoiceField
+                field_kwargs = {}
+
+        return field_cls(
             choices=choices,
             required=required,
+            # See #4084 - form builder bug causes empty option to be added. allow_blank
+            # is therefore required for select with `multiple: true` too.
             allow_blank=not required,
+            **field_kwargs,
         )
 
 

src/openforms/formio/tests/validation/test_currency.py

@@ -91,3 +91,37 @@ def test_currency_with_plugin_validator(self):
                 is_valid, _ = validate_formio_data(component, {"foo": 1.5})
 
                 self.assertFalse(is_valid)
+
+    def test_currency_required_validation(self):
+        component: Component = {
+            "type": "currency",
+            "key": "foo",
+            "label": "Test",
+            "validate": {"required": True},
+        }
+
+        invalid_values = [
+            ({}, "required"),
+            ({"foo": None}, "null"),
+        ]
+
+        for data, error_code in invalid_values:
+            with self.subTest(data=data):
+                is_valid, errors = validate_formio_data(component, data)
+
+                self.assertFalse(is_valid)
+                self.assertIn(component["key"], errors)
+                error = extract_error(errors, component["key"])
+                self.assertEqual(error.code, error_code)
+
+    def test_currency_optional_allows_empty(self):
+        component: Component = {
+            "type": "currency",
+            "key": "foo",
+            "label": "Test",
+            "validate": {"required": False},
+        }
+
+        is_valid, _ = validate_formio_data(component, {"foo": None})
+
+        self.assertTrue(is_valid)

src/openforms/formio/tests/validation/test_select.py

@@ -12,9 +12,6 @@ def test_select_field_required_validation(self):
             "key": "foo",
             "label": "Test",
             "data": {
-                "url": "",
-                "json": "",
-                "custom": "",
                 "values": [
                     {
                         "label": "first",
@@ -27,14 +24,76 @@ def test_select_field_required_validation(self):
                         "openForms": {"translations": {}},
                     },
                 ],
-                "resource": "",
             },
             "validate": {"required": True},
         }
 
         invalid_values = [
             ({}, "required"),
             ({"foo": None}, "null"),
+            ({"foo": ["first"]}, "invalid_choice"),
+        ]
+
+        for data, error_code in invalid_values:
+            with self.subTest(data=data):
+                is_valid, errors = validate_formio_data(component, data)
+
+                self.assertFalse(is_valid)
+                self.assertIn(component["key"], errors)
+                error = extract_error(errors, component["key"])
+                self.assertEqual(error.code, error_code)
+
+    def test_optional_single_select(self):
+        component: SelectComponent = {
+            "type": "select",
+            "key": "foo",
+            "label": "Test",
+            "data": {
+                "values": [
+                    {
+                        "label": "A",
+                        "value": "a",
+                    },
+                    {
+                        "label": "B",
+                        "value": "b",
+                    },
+                ],
+            },
+            "validate": {"required": False},
+        }
+
+        is_valid, _ = validate_formio_data(component, {"foo": ""})
+
+        self.assertTrue(is_valid)
+
+    def test_required_multiple_select(self):
+        component: SelectComponent = {
+            "type": "select",
+            "key": "foo",
+            "label": "Test",
+            "multiple": True,
+            "data": {
+                "values": [
+                    {
+                        "label": "A",
+                        "value": "a",
+                    },
+                    {
+                        "label": "B",
+                        "value": "b",
+                    },
+                ],
+            },
+            "validate": {"required": True},
+        }
+
+        invalid_values = [
+            ({}, "required"),
+            ({"foo": None}, "null"),
+            ({"foo": []}, "empty"),
+            ({"foo": [""]}, "invalid_choice"),
+            ({"foo": "b"}, "not_a_list"),
         ]
 
         for data, error_code in invalid_values: