我正在上一门关于Django的在线课程。在这门课程中，我将我的项目与Postgresql数据库连接。对于我的Django应用程序在Postgresql数据库启动之前启动的情况，我们编写了一个命令，将应用程序的启动延迟到Postgresql启动。下面是命令代码：

from psycopg2 import OperationalError as Psycopg2OpError

from django.db.utils import OperationalError
from django.core.management.base import BaseCommand

class Command(BaseCommand):

    def handle(self, *args, **options):
        """ Entrypoint for command """
        self.stdout.write('Waiting for database...')
        db_up = False
        while db_up is False:
            try:
                self.check(databases=['default'])
                db_up = True
            except(Psycopg2OpError, OperationalError):
                self.stdout.write('Database unavailable, waiting for 1 second...')
                time.sleep(1)

        self.stdout.write(self.style.SUCCESS('Database available!'))

但是我不明白测试这个命令是否有效的测试代码。你能解释一下这些代码的逻辑吗？

from unittest.mock import patch

from psycopg2 import OperationalError as Psycopg2Error

from django.core.management import call_command
from django.db.utils import OperationalError
from django.test import SimpleTestCase

@patch('core.management.commands.wait_for_db.Command.check')
class CommandTests(SimpleTestCase):
    """ Test commands """

    def test_wait_for_db_ready(self, patched_check):
        """ Test waiting for database if database is ready"""
        patched_check.return_value = True

        call_command('wait_for_db')

        patched_check.assert_called_once_with(databases=['default'])

    @patch('time.sleep')
    def test_wait_for_db_delay(self, patched_sleep, patched_check):
        """ Test waiting for database when getting OperationalError"""

        patched_check.side_effect = [Psycopg2Error] * 2 + \
            [OperationalError] * 3 + [True]

        call_command('wait_for_db')

        self.assertEqual(patched_check.call_count, 6)
        patched_check.assert_called_with(databases=['default'])