在《python编程呢个：从入门到实践》一书创建网页的应用程序时需要定义模型如下：
from django.db import models

class Topic(models.Model):
    """用户学习的主题"""
    text = models.CharField(max_length=200)
    date_added = models.DateTimeField(auto_now_add=True)

    def __str__(self):
        """返回模型的字符串表示"""
        return self.text
但是运行的时候出现如下错误：
Traceback (most recent call last):
  File "D:/pycharm/learning_log/models.py", line 3, in <module>
    class Topic(models.Model):
  File "D:\pycharm\learning_log\django\db\models\base.py", line 87, in __new__
    app_config = apps.get_containing_app_config(module)
  File "D:\pycharm\learning_log\django\apps\registry.py", line 249, in get_containing_app_config
    self.check_apps_ready()
  File "D:\pycharm\learning_log\django\apps\registry.py", line 131, in check_apps_ready
    settings.INSTALLED_APPS
  File "D:\pycharm\learning_log\django\conf\__init__.py", line 57, in __getattr__
    self._setup(name)
  File "D:\pycharm\learning_log\django\conf\__init__.py", line 42, in _setup
    % (desc, ENVIRONMENT_VARIABLE))
django.core.exceptions.ImproperlyConfigured: Requested setting INSTALLED_APPS, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.

Process finished with exit code 1


求教大神赐教如何解决！！！