这就是我一直在做的事情，我想这不是正确的解决方案：
- 修改我的模型
- 删除
db.sqlite3文件 makemigrations然后启动migrate
如果我在尝试运行时不删除我拥有的文件，那么：db.sqlite3makemigrationsmigrate
manage.py@pyweb > makemigrations
"C:\Program Files (x86)\JetBrains\PyCharm 4.5.3\bin\runnerw.exe" C:\Python34\python.exe "C:\Program Files (x86)\JetBrains\PyCharm 4.5.3\helpers\pycharm\django_manage.py" makemigrations C:/Users/Olivier/PycharmProjects/pyweb
No changes detected
Process finished with exit code 0
manage.py@pyweb > migrate
"C:\Program Files (x86)\JetBrains\PyCharm 4.5.3\bin\runnerw.exe" C:\Python34\python.exe "C:\Program Files (x86)\JetBrains\PyCharm 4.5.3\helpers\pycharm\django_manage.py" migrate C:/Users/Olivier/PycharmProjects/pyweb
Operations to perform:
Synchronize unmigrated apps: staticfiles, messages, produits
Apply all migrations: sessions, auth, contenttypes, admin
Synchronizing apps without migrations:
Creating tables...
Running deferred SQL...
Installing custom SQL...
Running migrations:
No migrations to apply.
Process finished with exit code 0
为了避免删除 db 文件，我缺少什么步骤？