使用sqlalchemy的时候

from flaskext.sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///models.db'

db = SQLAlchemy() #把这两行换成 db = SQLAlchemy(app)就正确了
db.init_app(app)

class Role(db.Model):
...

有这样的错误。。 为什么呢
raise RuntimeError('application not registered on db '
RuntimeError: application not registered on db instance and no application bound to current context