Add 'doc_pk' to PAPERLESS_FILENAME_FORMAT handling (#3861)

* Add 'doc_pk' to PAPERLESS_FILENAME_FORMAT handling

* Add test for 'doc_pk' filename formatter

docs/advanced_usage.md

@@ -311,6 +311,7 @@ Paperless provides the following placeholders within filenames:
 - `{added_day}`: Day added only (number 01-31).
 - `{owner_username}`: Username of document owner, if any, or "none"
 - `{original_name}`: Document original filename, minus the extension, if any, or "none"
+- `{doc_pk}`: The paperless identifier (primary key) for the document.
 
 Paperless will try to conserve the information from your database as
 much as possible. However, some characters that you can use in document

src/documents/file_handling.py

@@ -218,6 +218,7 @@ def generate_filename(
                 tag_list=tag_list,
                 owner_username=owner_username_str,
                 original_name=original_name,
+                doc_pk=f"{doc.pk:07}",
             ).strip()
 
             if settings.FILENAME_FORMAT_REMOVE_NONE:

src/documents/tests/test_file_handling.py

@@ -446,6 +446,19 @@ class TestFileHandling(DirectoriesMixin, FileSystemAssertsMixin, TestCase):
         self.assertIsNotDir(os.path.join(settings.ORIGINALS_DIR, "none"))
         self.assertIsDir(settings.ORIGINALS_DIR)
 
+    @override_settings(FILENAME_FORMAT="{doc_pk}")
+    def test_format_doc_pk(self):
+        document = Document()
+        document.pk = 1
+        document.mime_type = "application/pdf"
+        document.storage_type = Document.STORAGE_TYPE_UNENCRYPTED
+
+        self.assertEqual(generate_filename(document), "0000001.pdf")
+
+        document.pk = 13579
+
+        self.assertEqual(generate_filename(document), "0013579.pdf")
+
     @override_settings(FILENAME_FORMAT=None)
     def test_format_none(self):
         document = Document()