Chore: Run read-only migration tests through one migration cycle per class

Each TestMigrations test migrates back, seeds, migrates forward and returns to
the latest migration, which is slow. The fulltext query
prefix, sha256 checksum and skip archive file classes only read the migrated
data, so they now opt in to migrate_once: the first test runs the migration,
the rest reuse its state, and the schema is restored and the tables flushed in
tearDownClass.

src/documents/tests/search/test_migration_fulltext_query_field_prefixes.py

@@ -8,6 +8,7 @@ pytestmark = pytest.mark.search
 class TestMigrateFulltextQueryFieldPrefixes(TestMigrations):
     migrate_from = "0016_sha256_checksums"
     migrate_to = "0017_migrate_fulltext_query_field_prefixes"
+    migrate_once = True
 
     def setUpBeforeMigration(self, apps) -> None:
         User = apps.get_model("auth", "User")

src/documents/tests/test_migration_sha256_checksums.py

@@ -1,5 +1,4 @@
 import hashlib
-import shutil
 import tempfile
 from pathlib import Path
 
@@ -19,19 +18,22 @@ class TestSha256ChecksumDataMigration(TestMigrations):
 
     migrate_from = "0015_document_version_index_and_more"
     migrate_to = "0016_sha256_checksums"
+    migrate_once = True
     reset_sequences = True
 
     ORIGINAL_CONTENT = b"original file content for sha256 migration test"
     ARCHIVE_CONTENT = b"archive file content for sha256 migration test"
 
-    def setUpBeforeMigration(self, apps) -> None:
-        self._originals_dir = Path(tempfile.mkdtemp())
-        self._archive_dir = Path(tempfile.mkdtemp())
-        self._settings_override = override_settings(
-            ORIGINALS_DIR=self._originals_dir,
-            ARCHIVE_DIR=self._archive_dir,
+    @classmethod
+    def setUpClass(cls) -> None:
+        super().setUpClass()
+        originals_dir = Path(cls.enterClassContext(tempfile.TemporaryDirectory()))
+        archive_dir = Path(cls.enterClassContext(tempfile.TemporaryDirectory()))
+        cls.enterClassContext(
+            override_settings(ORIGINALS_DIR=originals_dir, ARCHIVE_DIR=archive_dir),
         )
-        self._settings_override.enable()
+
+    def setUpBeforeMigration(self, apps) -> None:
         Document = apps.get_model("documents", "Document")
 
         # doc1: original file present, no archive
@@ -85,8 +87,9 @@ class TestSha256ChecksumDataMigration(TestMigrations):
             archive_checksum=None,
         ).pk
 
-    def _fixture_teardown(self) -> None:
-        super()._fixture_teardown()
+    @classmethod
+    def tearDownClass(cls) -> None:
+        super().tearDownClass()
         # Django's SQLite backend returns [] from sequence_reset_sql(), so
         # reset_sequences=True flushes rows but never clears sqlite_sequence.
         # Explicitly delete the entry so subsequent tests start from pk=1.
@@ -96,12 +99,6 @@ class TestSha256ChecksumDataMigration(TestMigrations):
                     "DELETE FROM sqlite_sequence WHERE name='documents_document'",
                 )
 
-    def tearDown(self) -> None:
-        super().tearDown()
-        self._settings_override.disable()
-        shutil.rmtree(self._originals_dir, ignore_errors=True)
-        shutil.rmtree(self._archive_dir, ignore_errors=True)
-
     def test_original_checksum_updated_to_sha256_when_file_exists(self) -> None:
         Document = self.apps.get_model("documents", "Document")
         doc = Document.objects.get(pk=self.doc1_id)

src/documents/tests/utils.py

@@ -159,6 +159,16 @@ class ConsumeTaskMixin:
 
 
 class TestMigrations(TransactionTestCase):
+    """Run a migration on seeded data, then let the tests inspect the result.
+
+    By default every test migrates back, seeds, migrates forward and returns to
+    the latest migration, which costs several seconds. A class whose tests only
+    read the migrated data can set ``migrate_once`` to pay that once per class:
+    the migration runs for the first test, the database is left alone between
+    tests, and it is restored and flushed when the class finishes. Such tests
+    must not write to the database.
+    """
+
     @property
     def app(self):
         return apps.get_containing_app_config(type(self).__module__).name
@@ -166,10 +176,38 @@ class TestMigrations(TransactionTestCase):
     migrate_from = None
     dependencies = None
     migrate_to = None
+    migrate_once = False
+
+    _once_owner: "TestMigrations | None" = None
+    _once_state: dict[str, Any] | None = None
+    _once_finishing = False
 
     def setUp(self) -> None:
         super().setUp()
 
+        cls = type(self)
+        if self.migrate_once:
+            if cls._once_state is not None:
+                vars(self).update(cls._once_state)
+                return
+            if cls._once_owner is not None:
+                raise RuntimeError(
+                    f"The migration in '{cls.__name__}' failed for an earlier test",
+                )
+            # Recorded before migrating so a failed migration is still restored
+            cls._once_owner = self
+            before = dict(vars(self))
+
+        self._migrate()
+
+        if self.migrate_once:
+            cls._once_state = {
+                name: value
+                for name, value in vars(self).items()
+                if name not in before or before[name] is not value
+            }
+
+    def _migrate(self) -> None:
         assert self.migrate_from and self.migrate_to, (
             f"TestCase '{type(self).__name__}' must define migrate_from and migrate_to properties"
         )
@@ -197,19 +235,45 @@ class TestMigrations(TransactionTestCase):
     def setUpBeforeMigration(self, apps) -> None:
         pass
 
+    def _migrate_to_latest(self) -> None:
+        executor = MigrationExecutor(connection)
+        executor.loader.build_graph()
+        targets = executor.loader.graph.leaf_nodes()
+        executor.migrate(targets)
+
     def tearDown(self) -> None:
         """
         Ensure the database schema is restored to the latest migration after
         each migration test, so subsequent tests run against HEAD.
         """
+        if self.migrate_once and not self._once_finishing:
+            return
         try:
-            executor = MigrationExecutor(connection)
-            executor.loader.build_graph()
-            targets = executor.loader.graph.leaf_nodes()
-            executor.migrate(targets)
+            self._migrate_to_latest()
         finally:
             super().tearDown()
 
+    def _fixture_teardown(self) -> None:
+        # Django flushes every table after each test, which would discard the
+        # data the remaining tests of a migrate_once class still need
+        if self.migrate_once and not self._once_finishing:
+            return
+        super()._fixture_teardown()
+
+    @classmethod
+    def tearDownClass(cls) -> None:
+        owner = cls._once_owner
+        try:
+            if owner is not None:
+                cls._once_finishing = True
+                owner.tearDown()
+                owner._fixture_teardown()
+        finally:
+            cls._once_owner = None
+            cls._once_state = None
+            cls._once_finishing = False
+            super().tearDownClass()
+
 
 class SampleDirMixin:
     SAMPLE_DIR = Path(__file__).parent / "samples"

src/paperless/tests/test_migration_replace_skip_archive_file.py

@@ -4,6 +4,7 @@ from documents.tests.utils import TestMigrations
 class TestMigrateSkipArchiveFile(TestMigrations):
     migrate_from = "0007_optimize_integer_field_sizes"
     migrate_to = "0008_replace_skip_archive_file"
+    migrate_once = True
 
     def setUpBeforeMigration(self, apps):
         ApplicationConfiguration = apps.get_model(