Added TransactionTestCase.available_apps.

This can be used to make Django's test suite significantly faster by
reducing the number of models for which content types and permissions
must be created and tables must be flushed in each non-transactional
test.

It's documented for Django contributors and committers but it's branded
as a private API to preserve our freedom to change it in the future.

Most of the credit goes to Anssi. He got the idea and did the research.

Fixed #20483.

django/test/testcases.py

@@ -28,6 +28,7 @@ from django.core.servers.basehttp import (WSGIRequestHandler, WSGIServer,
     WSGIServerException)
 from django.core.urlresolvers import clear_url_caches, set_urlconf
 from django.db import connection, connections, DEFAULT_DB_ALIAS, transaction
+from django.db.models.loading import cache
 from django.forms.fields import CharField
 from django.http import QueryDict
 from django.test.client import Client
@@ -725,6 +726,9 @@ class TransactionTestCase(SimpleTestCase):
     # test case
     reset_sequences = False
 
+    # Subclasses can enable only a subset of apps for faster tests
+    available_apps = None
+
     def _pre_setup(self):
         """Performs any pre-test setup. This includes:
 
@@ -733,7 +737,14 @@ class TransactionTestCase(SimpleTestCase):
              named fixtures.
         """
         super(TransactionTestCase, self)._pre_setup()
-        self._fixture_setup()
+        if self.available_apps is not None:
+            cache.set_available_apps(self.available_apps)
+        try:
+            self._fixture_setup()
+        except Exception:
+            if self.available_apps is not None:
+                cache.unset_available_apps()
+            raise
 
     def _databases_names(self, include_mirrors=True):
         # If the test case has a multi_db=True flag, act on all databases,
@@ -775,22 +786,27 @@ class TransactionTestCase(SimpleTestCase):
            * Force closing the connection, so that the next test gets
              a clean cursor.
         """
-        self._fixture_teardown()
-        super(TransactionTestCase, self)._post_teardown()
-        # Some DB cursors include SQL statements as part of cursor
-        # creation. If you have a test that does rollback, the effect
-        # of these statements is lost, which can effect the operation
-        # of tests (e.g., losing a timezone setting causing objects to
-        # be created with the wrong time).
-        # To make sure this doesn't happen, get a clean connection at the
-        # start of every test.
-        for conn in connections.all():
-            conn.close()
+        try:
+            self._fixture_teardown()
+            super(TransactionTestCase, self)._post_teardown()
+            # Some DB cursors include SQL statements as part of cursor
+            # creation. If you have a test that does rollback, the effect of
+            # these statements is lost, which can effect the operation of
+            # tests (e.g., losing a timezone setting causing objects to be
+            # created with the wrong time). To make sure this doesn't happen,
+            # get a clean connection at the start of every test.
+            for conn in connections.all():
+                conn.close()
+        finally:
+            cache.unset_available_apps()
 
     def _fixture_teardown(self):
+        # Allow TRUNCATE ... CASCADE when flushing only a subset of the apps
+        allow_cascade = self.available_apps is not None
         for db_name in self._databases_names(include_mirrors=False):
-            call_command('flush', verbosity=0, interactive=False, database=db_name,
-                         skip_validation=True, reset_sequences=False)
+            call_command('flush', verbosity=0, interactive=False,
+                         database=db_name, skip_validation=True,
+                         reset_sequences=False, allow_cascade=allow_cascade)
 
     def assertQuerysetEqual(self, qs, values, transform=repr, ordered=True):
         items = six.moves.map(transform, qs)