We need to use global objects here

src/documents/bulk_edit.py

@@ -638,7 +638,7 @@ def merge_as_versions(
             raise ValueError(
                 "A version label can only be set when merging one source document.",
             )
-        if Document.objects.filter(root_document_id__in=source_ids).exists():
+        if Document.global_objects.filter(root_document_id__in=source_ids).exists():
             raise ValueError(
                 "Documents with existing versions cannot be merged into another document.",
             )

src/documents/serialisers.py

@@ -1717,7 +1717,7 @@ class MergeDocumentsAsVersionsSerializer(DocumentListSerializer):
             )
 
         source_document_ids = set(documents) - {attrs["root_document_id"]}
-        if Document.objects.filter(
+        if Document.global_objects.filter(
             root_document_id__in=source_document_ids,
         ).exists():
             raise serializers.ValidationError(

src/documents/tests/test_merge_documents_as_versions.py

@@ -141,6 +141,27 @@ class TestMergeDocumentsAsVersionsSerializer(TestCase):
             "Documents with existing versions cannot be merged into another document.",
         )
 
+    def test_rejects_source_document_with_trashed_versions(self) -> None:
+        version = Document.objects.create(
+            checksum="D",
+            title="D",
+            root_document=self.doc1,
+            version_index=1,
+        )
+        version.delete()  # trashed, but still points at doc1
+        serializer = MergeDocumentsAsVersionsSerializer(
+            data={
+                "documents": [self.doc1.id, self.doc2.id],
+                "root_document_id": self.doc2.id,
+            },
+        )
+
+        self.assertFalse(serializer.is_valid())
+        self.assertEqual(
+            serializer.errors["non_field_errors"][0],
+            "Documents with existing versions cannot be merged into another document.",
+        )
+
     def test_allows_root_document_with_versions(self) -> None:
         Document.objects.create(
             checksum="D",
@@ -268,6 +289,42 @@ class TestMergeDocumentsAsVersions(TestCase):
         bulk_update_mock.assert_not_called()
         status_manager_mock.assert_not_called()
 
+    @mock.patch("documents.bulk_edit.DocumentsStatusManager")
+    @mock.patch("documents.bulk_edit.bulk_update_documents.apply_async")
+    @mock.patch("documents.bulk_edit.remove_document_from_index.apply_async")
+    def test_rejects_source_document_with_trashed_versions(
+        self,
+        remove_from_index_mock,
+        bulk_update_mock,
+        status_manager_mock,
+    ) -> None:
+        source = Document.objects.create(checksum="A", title="Source")
+        version = Document.objects.create(
+            checksum="B",
+            title="Source version",
+            root_document=source,
+            version_index=1,
+        )
+        version.delete()  # trashed, but still points at source
+        root = Document.objects.create(checksum="C", title="Root")
+
+        with self.assertRaisesRegex(ValueError, "existing versions"):
+            merge_as_versions(
+                [source.id, root.id],
+                root_document_id=root.id,
+            )
+
+        source.refresh_from_db()
+        self.assertIsNone(source.root_document_id)
+        # Restoring the version must not produce a version of a version
+        self.assertEqual(
+            Document.global_objects.get(pk=version.pk).root_document_id,
+            source.id,
+        )
+        remove_from_index_mock.assert_not_called()
+        bulk_update_mock.assert_not_called()
+        status_manager_mock.assert_not_called()
+
 
 class TestMergeDocumentsAsVersionsAPI(APITestCase):
     def setUp(self) -> None: