Also fix this setting false/0 from the frontend

docs/usage.md

@@ -576,7 +576,9 @@ The following workflow action types are available:
 - Tags, correspondent, document type and storage path
 - Document owner
 - View and / or edit permissions to users or groups
-- Custom fields. Note that no value for the field will be set
+- Custom fields, optionally with a value. If no value is set, the field is only added to the
+  document and any value it may already have is left untouched. If a value is set, it will
+  overwrite an existing value of that field on the document.
 
 ##### Removal {#workflow-action-removal}
 

src-ui/src/app/components/common/input/custom-fields-values/custom-fields-values.component.spec.ts

@@ -52,10 +52,10 @@ describe('CustomFieldsValuesComponent', () => {
   })
 
   it('should set selectedFields and map values correctly', () => {
-    component.value = { 1: 'value1' }
-    component.selectedFields = [1, 2]
-    expect(component.selectedFields).toEqual([1, 2])
-    expect(component.value).toEqual({ 1: 'value1', 2: null })
+    component.value = { 1: 'value1', 3: 0, 4: false }
+    component.selectedFields = [1, 2, 3, 4]
+    expect(component.selectedFields).toEqual([1, 2, 3, 4])
+    expect(component.value).toEqual({ 1: 'value1', 2: null, 3: 0, 4: false })
   })
 
   it('should return the correct custom field by id', () => {

src-ui/src/app/components/common/input/custom-fields-values/custom-fields-values.component.ts

@@ -77,7 +77,7 @@ export class CustomFieldsValuesComponent extends AbstractInputComponent<Object>
     this._selectedFields = newFields
     // map the selected fields to an object with field_id as key and value as value
     this.value = newFields.reduce((acc, fieldId) => {
-      acc[fieldId] = this.value?.[fieldId] || null
+      acc[fieldId] = this.value?.[fieldId] ?? null
       return acc
     }, {})
     this.onChange(this.value)

src/documents/serialisers.py

@@ -3213,6 +3213,13 @@ class WorkflowActionSerializer(serializers.ModelSerializer[WorkflowAction]):
                         {"assign_title": f'Invalid f-string detected: "{e.args[0]}"'},
                     )
 
+        if attrs.get("assign_custom_fields_values"):
+            # Empty strings treated as None to avoid unexpected behavior
+            attrs["assign_custom_fields_values"] = {
+                field_id: (None if value == "" else value)
+                for field_id, value in attrs["assign_custom_fields_values"].items()
+            }
+
         if (
             "type" in attrs
             and attrs["type"] == WorkflowAction.WorkflowActionType.EMAIL

src/documents/tests/test_api_workflows.py

@@ -422,6 +422,11 @@ class TestApiWorkflows(DirectoriesMixin, APITestCase):
             json.dumps(
                 {
                     "assign_title": "",
+                    "assign_custom_fields": [self.cf1.id, self.cf2.id],
+                    "assign_custom_fields_values": {
+                        str(self.cf1.id): "",
+                        str(self.cf2.id): 0,
+                    },
                 },
             ),
             content_type="application/json",
@@ -429,6 +434,10 @@ class TestApiWorkflows(DirectoriesMixin, APITestCase):
         self.assertEqual(response.status_code, status.HTTP_201_CREATED)
         action = WorkflowAction.objects.get(id=response.data["id"])
         self.assertIsNone(action.assign_title)
+        self.assertEqual(
+            action.assign_custom_fields_values,
+            {str(self.cf1.id): None, str(self.cf2.id): 0},
+        )
 
         response = self.client.post(
             self.ENDPOINT_TRIGGERS,

src/documents/tests/test_workflows.py

@@ -2000,6 +2000,55 @@ class TestWorkflows(
                 r"Doc added in \w{3,}",
             )  # Match any 3-letter month name
 
+    def test_document_updated_workflow_existing_custom_field_empty_value(self) -> None:
+        """
+        GIVEN:
+            - Existing workflow with UPDATED trigger and action that assigns a custom field
+              with an empty value
+        WHEN:
+            - Document is updated that already contains the field with a value
+        THEN:
+            - The existing value is left untouched, see GH #13627
+        """
+        trigger = WorkflowTrigger.objects.create(
+            type=WorkflowTrigger.WorkflowTriggerType.DOCUMENT_UPDATED,
+            filter_has_document_type=self.dt,
+        )
+        action = WorkflowAction.objects.create()
+        action.assign_custom_fields.add(self.cf1)
+        action.assign_custom_fields_values = {self.cf1.pk: ""}
+        action.save()
+        w = Workflow.objects.create(
+            name="Workflow 1",
+            order=0,
+        )
+        w.triggers.add(trigger)
+        w.actions.add(action)
+        w.save()
+
+        doc = Document.objects.create(
+            title="sample test",
+            correspondent=self.c,
+            original_filename="sample.pdf",
+        )
+        CustomFieldInstance.objects.create(
+            document=doc,
+            field=self.cf1,
+            value_text="existing value",
+        )
+
+        superuser = User.objects.create_superuser("superuser")
+        self.client.force_authenticate(user=superuser)
+
+        self.client.patch(
+            f"/api/documents/{doc.id}/",
+            {"document_type": self.dt.id},
+            format="json",
+        )
+
+        doc.refresh_from_db()
+        self.assertEqual(doc.custom_fields.get(field=self.cf1).value, "existing value")
+
     def test_document_updated_workflow_existing_custom_field(self) -> None:
         """
         GIVEN:

src/documents/workflows/mutations.py

@@ -105,7 +105,8 @@ def apply_assignment_to_document(
                 field=field,
                 document=document,
             ).first()
-            if instance and args[value_field_name] is not None:
+            # empty string is indistinguishable from no value in the UI
+            if instance and args[value_field_name] not in (None, ""):
                 setattr(instance, value_field_name, args[value_field_name])
                 instance.save()
             elif not instance: