management: Use Realm.objects.get instead of get_realm.

The Realm.DoesNotExist exception will never be raised if we pass a
string_id as in that case zerver.models.get_realm function would be
used for fetching. realm.zerver.models.get_realm uses filter query
so the exception will not be raised even if there are no realm which
matches the query.

Tests added by tabbott.

zerver/lib/management.py

@@ -7,7 +7,7 @@ from django.core.exceptions import MultipleObjectsReturned
 from django.core.management.base import BaseCommand, CommandError
 from typing import Any, Dict, Optional, Text
 
-from zerver.models import get_realm, Realm, UserProfile
+from zerver.models import Realm, UserProfile
 
 def is_integer_string(val):
     # type: (str) -> bool
@@ -39,7 +39,7 @@ class ZulipBaseCommand(BaseCommand):
         try:
             if is_integer_string(val):
                 return Realm.objects.get(id=val)
-            return get_realm(val)
+            return Realm.objects.get(string_id=val)
         except Realm.DoesNotExist:
             raise CommandError("The is no realm with id '%s'. Aborting." %
                                (options["realm_id"],))

zerver/tests/test_management_commands.py

@@ -25,6 +25,8 @@ class TestZulipBaseCommand(ZulipTestCase):
         self.assertEqual(command.get_realm(dict(realm_id='1')), get_realm("zulip"))
         with self.assertRaisesRegex(CommandError, "The is no realm with id"):
             command.get_realm(dict(realm_id='17'))
+        with self.assertRaisesRegex(CommandError, "The is no realm with id"):
+            command.get_realm(dict(realm_id='mit'))
 
     def test_get_user(self):
         # type: () -> None