Fix validate document link targets

src/documents/tests/test_api_bulk_edit.py

@@ -262,6 +262,50 @@ class TestBulkEditAPI(DirectoriesMixin, APITestCase):
         self.assertEqual(kwargs["add_custom_fields"], [self.cf1.id])
         self.assertEqual(kwargs["remove_custom_fields"], [self.cf2.id])
 
+    @mock.patch("documents.serialisers.bulk_edit.modify_custom_fields")
+    def test_api_modify_custom_fields_documentlink_forbidden_for_unpermitted_target(
+        self,
+        m,
+    ):
+        self.setup_mock(m, "modify_custom_fields")
+        user = User.objects.create_user(username="doc-owner")
+        user.user_permissions.add(Permission.objects.get(codename="change_document"))
+        other_user = User.objects.create_user(username="other-user")
+        source_doc = Document.objects.create(
+            checksum="source",
+            title="Source",
+            owner=user,
+        )
+        target_doc = Document.objects.create(
+            checksum="target",
+            title="Target",
+            owner=other_user,
+        )
+        doclink_field = CustomField.objects.create(
+            name="doclink",
+            data_type=CustomField.FieldDataType.DOCUMENTLINK,
+        )
+
+        self.client.force_authenticate(user=user)
+
+        response = self.client.post(
+            "/api/documents/bulk_edit/",
+            json.dumps(
+                {
+                    "documents": [source_doc.id],
+                    "method": "modify_custom_fields",
+                    "parameters": {
+                        "add_custom_fields": {doclink_field.id: [target_doc.id]},
+                        "remove_custom_fields": [],
+                    },
+                },
+            ),
+            content_type="application/json",
+        )
+
+        self.assertEqual(response.status_code, status.HTTP_403_FORBIDDEN)
+        m.assert_not_called()
+
     @mock.patch("documents.serialisers.bulk_edit.modify_custom_fields")
     def test_api_modify_custom_fields_with_values(self, m):
         self.setup_mock(m, "modify_custom_fields")

src/documents/tests/test_api_custom_fields.py

@@ -6,6 +6,7 @@ from unittest.mock import ANY
 from django.contrib.auth.models import Permission
 from django.contrib.auth.models import User
 from django.test import override_settings
+from guardian.shortcuts import assign_perm
 from rest_framework import status
 from rest_framework.test import APITestCase
 
@@ -1247,6 +1248,100 @@ class TestCustomFieldsAPI(DirectoriesMixin, APITestCase):
         self.assertEqual(resp.status_code, status.HTTP_200_OK)
         self.assertEqual(doc5.custom_fields.first().value, [1])
 
+    def test_documentlink_patch_requires_change_permission_on_target_documents(self):
+        source_owner = User.objects.create_user(username="source-owner")
+        source_owner.user_permissions.add(
+            Permission.objects.get(codename="change_document"),
+        )
+        other_user = User.objects.create_user(username="other-user")
+
+        source_doc = Document.objects.create(
+            title="Source",
+            checksum="source",
+            mime_type="application/pdf",
+            owner=source_owner,
+        )
+        target_doc = Document.objects.create(
+            title="Target",
+            checksum="target",
+            mime_type="application/pdf",
+            owner=other_user,
+        )
+        custom_field_doclink = CustomField.objects.create(
+            name="Test Custom Field Doc Link",
+            data_type=CustomField.FieldDataType.DOCUMENTLINK,
+        )
+
+        self.client.force_authenticate(user=source_owner)
+
+        resp = self.client.patch(
+            f"/api/documents/{source_doc.id}/",
+            data={
+                "custom_fields": [
+                    {
+                        "field": custom_field_doclink.id,
+                        "value": [target_doc.id],
+                    },
+                ],
+            },
+            format="json",
+        )
+
+        self.assertEqual(resp.status_code, status.HTTP_403_FORBIDDEN)
+        self.assertEqual(
+            CustomFieldInstance.objects.filter(field=custom_field_doclink).count(),
+            0,
+        )
+
+    def test_documentlink_patch_allowed_with_change_permission_on_target_documents(
+        self,
+    ):
+        source_owner = User.objects.create_user(username="source-owner")
+        source_owner.user_permissions.add(
+            Permission.objects.get(codename="change_document"),
+        )
+        other_user = User.objects.create_user(username="other-user")
+
+        source_doc = Document.objects.create(
+            title="Source",
+            checksum="source",
+            mime_type="application/pdf",
+            owner=source_owner,
+        )
+        target_doc = Document.objects.create(
+            title="Target",
+            checksum="target",
+            mime_type="application/pdf",
+            owner=other_user,
+        )
+        custom_field_doclink = CustomField.objects.create(
+            name="Test Custom Field Doc Link",
+            data_type=CustomField.FieldDataType.DOCUMENTLINK,
+        )
+
+        assign_perm("change_document", source_owner, target_doc)
+        self.client.force_authenticate(user=source_owner)
+
+        resp = self.client.patch(
+            f"/api/documents/{source_doc.id}/",
+            data={
+                "custom_fields": [
+                    {
+                        "field": custom_field_doclink.id,
+                        "value": [target_doc.id],
+                    },
+                ],
+            },
+            format="json",
+        )
+
+        self.assertEqual(resp.status_code, status.HTTP_200_OK)
+        target_doc.refresh_from_db()
+        self.assertEqual(
+            target_doc.custom_fields.get(field=custom_field_doclink).value,
+            [source_doc.id],
+        )
+
     def test_custom_field_filters(self):
         custom_field_string = CustomField.objects.create(
             name="Test Custom Field String",