Also catch invalid f-string at serializer

And display in workflow edit modal

src-ui/src/app/components/common/edit-dialog/workflow-edit-dialog/workflow-edit-dialog.component.html

@@ -99,7 +99,7 @@
                         <input type="hidden" formControlName="id" />
                         <div class="row">
                           <div class="col">
-                            <pngx-input-text i18n-title title="Assign title" formControlName="assign_title" i18n-hint hint="Can include some placeholders, see <a target='_blank' href='https://docs.paperless-ngx.com/usage/#workflows'>documentation</a>." [error]="error?.assign_title"></pngx-input-text>
+                            <pngx-input-text i18n-title title="Assign title" formControlName="assign_title" i18n-hint hint="Can include some placeholders, see <a target='_blank' href='https://docs.paperless-ngx.com/usage/#workflows'>documentation</a>." [error]="error?.actions?.[i]?.assign_title"></pngx-input-text>
                             <pngx-input-tags [allowCreate]="false" i18n-title title="Assign tags" formControlName="assign_tags"></pngx-input-tags>
                             <pngx-input-select i18n-title title="Assign document type" [items]="documentTypes" [allowNull]="true" formControlName="assign_document_type"></pngx-input-select>
                             <pngx-input-select i18n-title title="Assign correspondent" [items]="correspondents" [allowNull]="true" formControlName="assign_correspondent"></pngx-input-select>

src/documents/serialisers.py

@@ -1386,12 +1386,38 @@ class WorkflowActionSerializer(serializers.ModelSerializer):
 
     def validate(self, attrs):
         # Empty strings treated as None to avoid unexpected behavior
-        if (
+        if "assign_title" in attrs:
-            "assign_title" in attrs
+            if attrs["assign_title"] is not None and len(attrs["assign_title"]) == 0:
-            and attrs["assign_title"] is not None
-            and len(attrs["assign_title"]) == 0
-        ):
                 attrs["assign_title"] = None
+            else:
+                try:
+                    # test against all placeholders, see consumer.py `parse_doc_title_w_placeholders`
+                    attrs["assign_title"].format(
+                        correspondent="",
+                        document_type="",
+                        added="",
+                        added_year="",
+                        added_year_short="",
+                        added_month="",
+                        added_month_name="",
+                        added_month_name_short="",
+                        added_day="",
+                        added_time="",
+                        owner_username="",
+                        original_filename="",
+                        created="",
+                        created_year="",
+                        created_year_short="",
+                        created_month="",
+                        created_month_name="",
+                        created_month_name_short="",
+                        created_day="",
+                        created_time="",
+                    )
+                except ValueError as e:
+                    raise serializers.ValidationError(
+                        {"assign_title": f"Invalid f-string detected: {e.args[0]}"},
+                    )
 
         return attrs
 

src/documents/tests/test_api_workflows.py

@@ -248,6 +248,45 @@ class TestApiWorkflows(DirectoriesMixin, APITestCase):
 
         self.assertEqual(WorkflowTrigger.objects.count(), 1)
 
+    def test_api_create_invalid_assign_title(self):
+        """
+        GIVEN:
+            - API request to create a workflow
+            - Invalid f-string for assign_title
+        WHEN:
+            - API is called
+        THEN:
+            - Correct HTTP 400 response
+            - No objects are created
+        """
+        response = self.client.post(
+            self.ENDPOINT,
+            json.dumps(
+                {
+                    "name": "Workflow 1",
+                    "order": 1,
+                    "triggers": [
+                        {
+                            "type": WorkflowTrigger.WorkflowTriggerType.DOCUMENT_UPDATED,
+                        },
+                    ],
+                    "actions": [
+                        {
+                            "assign_title": "{created_year]",
+                        },
+                    ],
+                },
+            ),
+            content_type="application/json",
+        )
+        self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST)
+        self.assertIn(
+            "Invalid f-string detected",
+            response.data["actions"][0]["assign_title"][0],
+        )
+
+        self.assertEqual(Workflow.objects.count(), 1)
+
     def test_api_create_workflow_trigger_action_empty_fields(self):
         """
         GIVEN: