[1.7.x] Fixed #23975 -- Restored pre_migrate signal if all apps have migrations.

Thanks kmmbvnr for the report.

Backport of d2ff8a7241 from master

django/core/management/commands/migrate.py

@@ -128,6 +128,7 @@ class Command(BaseCommand):
             created_models = self.sync_apps(connection, executor.loader.unmigrated_apps)
         else:
             created_models = []
+            emit_pre_migrate_signal([], self.verbosity, self.interactive, connection.alias)
 
         # The test runner requires us to flush after a syncdb but before migrations,
         # so do that here.

docs/releases/1.7.2.txt

@@ -146,3 +146,6 @@ Bugfixes
 
 * Fixed migration crash when adding ``order_with_respect_to`` to a table
   with existing rows (:ticket:`23983`).
+
+* Restored the ``pre_migrate`` signal if all apps have migrations
+  (:ticket:`23975`).

tests/migrate_signals/tests.py

@@ -1,7 +1,7 @@
 from django.apps import apps
 from django.core import management
 from django.db.models import signals
-from django.test import TestCase
+from django.test import override_settings, TestCase
 from django.test.utils import override_system_checks
 from django.utils import six
 
@@ -79,3 +79,23 @@ class MigrateSignalTests(TestCase):
         self.assertEqual(args['verbosity'], MIGRATE_VERBOSITY)
         self.assertEqual(args['interactive'], MIGRATE_INTERACTIVE)
         self.assertEqual(args['using'], 'default')
+
+    @override_system_checks([])
+    @override_settings(MIGRATION_MODULES={'migrate_signals': 'migrate_signals.custom_migrations'})
+    def test_pre_migrate_migrations_only(self):
+        """
+        If all apps have migrations, pre_migrate should be sent.
+        """
+        r = PreMigrateReceiver()
+        signals.pre_migrate.connect(r, sender=APP_CONFIG)
+        stdout = six.StringIO()
+        management.call_command('migrate', database=MIGRATE_DATABASE,
+            verbosity=MIGRATE_VERBOSITY, interactive=MIGRATE_INTERACTIVE,
+            load_initial_data=False, stdout=stdout)
+        args = r.call_args
+        self.assertEqual(r.call_counter, 1)
+        self.assertEqual(set(args), set(PRE_MIGRATE_ARGS))
+        self.assertEqual(args['app_config'], APP_CONFIG)
+        self.assertEqual(args['verbosity'], MIGRATE_VERBOSITY)
+        self.assertEqual(args['interactive'], MIGRATE_INTERACTIVE)
+        self.assertEqual(args['using'], 'default')