[#5039] Fix email registration errors

src/openforms/forms/tests/test_api_form_plugin_options.py

@@ -272,10 +272,4 @@ def test_cannot_overwrite_only_registration_email_html_template(self):
         data = response.json()
         error = data["invalidParams"][0]["reason"]
 
-        self.assertEqual(
-            error,
-            _(
-                "The fields {fields} must all have a non-empty value as soon as one of them "
-                "does."
-            ).format(fields="email_content_template_html, email_content_template_text"),
-        )
+        self.assertEqual(error, _("You must also specify text along with the html."))

src/openforms/registrations/contrib/email/config.py

@@ -5,7 +5,6 @@
 from rest_framework import serializers
 from typing_extensions import NotRequired
 
-from openforms.api.validators import AllOrNoneTruthyFieldsValidator
 from openforms.emails.validators import URLSanitationValidator
 from openforms.formio.api.fields import FormioVariableKeyField
 from openforms.template.validators import DjangoTemplateValidator
@@ -123,13 +122,33 @@ class EmailOptionsSerializer(JsonSchemaSerializerMixin, serializers.Serializer):
         ],
     )
 
-    class Meta:
-        validators = [
-            AllOrNoneTruthyFieldsValidator(
-                "email_content_template_html",
-                "email_content_template_text",
-            ),
-        ]
+    def validate(self, attrs: Options) -> Options:
+        html = attrs.get("email_content_template_html")
+        text = attrs.get("email_content_template_text")
+
+        if not html and not text:
+            return attrs
+
+        if html and not text:
+            raise serializers.ValidationError(
+                {
+                    "email_content_template_text": _(
+                        "You must also specify text along with the html."
+                    ),
+                },
+                code="required",
+            )
+        if text and not html:
+            raise serializers.ValidationError(
+                {
+                    "email_content_template_html": _(
+                        "You must also specify html along with the text."
+                    ),
+                },
+                code="required",
+            )
+
+        return attrs
 
 
 # sanity check for development - keep serializer and type definitions in sync

src/openforms/registrations/contrib/email/tests/test_serializers.py

@@ -0,0 +1,48 @@
+from django.test import TestCase
+
+from ..config import EmailOptionsSerializer
+
+
+class EmailOptionsSerializerTest(TestCase):
+    def test_email_and_text_content_fields(self):
+        with self.subTest("both fields missing"):
+            serializer = EmailOptionsSerializer(
+                data={
+                    "to_emails": ["[email protected]"],
+                    "email_content_template_html": "",
+                    "email_content_template_text": "",
+                },
+                context={"request": None},
+            )
+
+            self.assertTrue(serializer.is_valid())
+
+        with self.subTest("text is missing"):
+            serializer = EmailOptionsSerializer(
+                data={
+                    "to_emails": ["[email protected]"],
+                    "email_content_template_html": "<p>some text</p>",
+                    "email_content_template_text": "",
+                },
+                context={"request": None},
+            )
+
+            self.assertFalse(serializer.is_valid())
+            self.assertEqual(
+                serializer.errors["email_content_template_text"][0].code, "required"
+            )
+
+        with self.subTest("html is missing"):
+            serializer = EmailOptionsSerializer(
+                data={
+                    "to_emails": ["[email protected]"],
+                    "email_content_template_html": "",
+                    "email_content_template_text": "some text",
+                },
+                context={"request": None},
+            )
+
+            self.assertFalse(serializer.is_valid())
+            self.assertEqual(
+                serializer.errors["email_content_template_html"][0].code, "required"
+            )